The article deals with the question of using the censuses carried out by the Civilian-Military Order Commission of the Cracow Voivodeship as a source for demographic research exemplified by the Proszowice County. The censuses were conducted in the years 1790–1792 and were not tax character, hence they are commonly considered to be more precise and reliable than the ones conducted earlier. The article presents the specific character of the sources of that kind and the resulting difficulties such as passing over part of the population, terminological complexities, unclear division into households. A lot of attention has been paid to the assessment of the credibility of historical sources, with such indicators as the percentage of the youngest children and old people in the population or the male sex ratio in newborn babies. The analyses of age heaping has been carried out with the use of the of Whipple’s and Myers’ indexes. The conclusions indicate that the censuses – especially in the parts concerning age analysis – turn out to be a problematic sources, yet some of their short comings might be overcome when appropriate statistical methods have been applied. Frequent age heaping is not decisive factors in the credibility of the census; what should be decisive is the analysis of the whole – the amount of the information in the census, diligence and accuracy, and compatibility with other sources, e.g. the Poniatowski census of 1787.