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Background/Objectives: The prognostic significance of alkalemia found in an initial emergency department (ED) evaluation has not been described thus far. Methods: We retrospectively reviewed the records of all patients aged 18 years or older evaluated in the ED of one large academic referral center during 2000-2023. Included patients were those with at least one measurement of pH ≥ 7.55 upon initial ED presentation. Alkalemia was deemed primarily metabolic (PM) if PCO 2 was ≥35 mmHg and primarily respiratory (PR) if bicarbonate levels were ≤24 mEq/L. The primary outcome was survival 30 days from ED presentation. Results: Of 2440 patients included, 199 (8.1%) had PM and 1494 (61.2%) had PR. Alkalemia severity was not correlated with prognosis. Survival at 30 days was significantly ( p < 0.001) lower in the PM group (78.9%) compared with that of either the PR (95.3%) or the combined etiology (92.2%) groups. Multivariate survival analysis after balancing potential observed confounders using propensity score matching revealed the type of alkalemia (PM vs. PR) to be a significant predictor of 30-day mortality (aHR 1.73; 95% C.I. = [1.07 to 2.82]; p = 0.026), irrespective of age, other laboratory values obtained on ED evaluation (including pH), past medical history, or vital signs on presentation. Conclusions: In patients presenting to the ED with significant alkalemia, the mechanism of alkalemia, i.e., primarily metabolic versus primarily respiratory, rather than the absolute degree of alkalemia, is associated with increased mortality.