Background In an effort to prevent continued increases in obesity and diabetes, in January 2014, the Mexican government implemented an 8% tax on nonessential foods with energy density ≥275 kcal/100 g and a peso-per-liter tax on sugar-sweetened beverages (SSBs). Limited rigorous evaluations of food taxes exist worldwide. The objective of this study was to examine changes in volume of taxed and untaxed packaged food purchases in response to these taxes in the entire sample and stratified by socioeconomic status (SES). Methods and Findings This study uses data on household packaged food purchases representative of the Mexican urban population from The Nielsen Company’s Mexico Consumer Panel Services (CPS). We included 6,248 households that participated in the Nielsen CPS in at least 2 mo during 2012–2014; average household follow-up was 32.7 mo. We analyzed the volume of purchases of taxed and untaxed foods from January 2012 to December 2014, using a longitudinal, fixed-effects model that adjusted for preexisting trends to test whether the observed post-tax trend was significantly different from the one expected based on the pre-tax trend. We controlled for household characteristics and contextual factors like minimum salary and unemployment rate. The mean volume of purchases of taxed foods in 2014 changed by -25 g (95% confidence interval = -46, -11) per capita per month, or a 5.1% change beyond what would have been expected based on pre-tax (2012–2013) trends, with no corresponding change in purchases of untaxed foods. Low SES households purchased on average 10.2% less taxed foods than expected (-44 [–72, –16] g per capita per month); medium SES households purchased 5.8% less taxed foods than expected (-28 [–46, –11] g per capita per month), whereas high SES households’ purchases did not change. The main limitations of our findings are the inability to infer causality because the taxes were implemented at the national level (lack of control group), our sample is only representative of urban areas, we only have 2 y of data prior to the tax, and, as with any consumer panel survey, we did not capture all foods purchased by the household. Conclusions Household purchases of nonessential energy-dense foods declined in the first year after the implementation of Mexico’s SSB and nonessential foods taxes. Future studies should evaluate the impact of the taxes on overall energy intake, dietary quality, and food purchase patterns (see S1 Abstract in Spanish)., An 8% tax on high-energy, non-essential foods in Mexico reduces the amount of these foods consumed, an effect that is more pronounced in lower income families, Taillie and colleagues reveal., Author Summary Why Was This Study Done? In January 2014, Mexico passed an 8% tax on nonessential foods with energy density ≥275 kcal/100 g, including salty snacks, chips, cakes, pastries, and frozen desserts; and a 1 peso/liter (~10%) tax on sugar-sweetened beverages.To date, there has been very limited research as to how larger health-related food/beverage taxes change household food purchases, or whether low socioeconomic status (SES) households are more responsive to such taxes. What Did the Researchers Do and Find? Using a dataset that follows household food purchases over time, we examined whether the volume of taxed foods showed greater declines in the post-tax period than we would have expected based on trends in the volume of taxed food purchases prior to the tax. We also examined whether post-tax changes in the volume of taxed food purchases was greater among low SES households.We found that the mean volume of purchases of taxed foods in 2014 declined by 25 g per capita per month, or a 5.1% change beyond what would have been expected based on pre-tax (2012–2013) trends.There were no changes in the purchase of untaxed foods in the post-tax period.Low SES households’ purchases of taxed foods declined by 10.2% and medium SES households by 5.8%, whereas high SES did not change. What Do These Findings Mean? These findings show that in the post-tax period, purchases of taxed foods declined more than we would expect if pre-tax trends had simply continued, particularly among low and medium SES households. Future research should explore how these shifts are linked to changes in the nutritional quality of the overall diet.