Metabolic and respiratory acid-base disorders occur as single and mixed entities. When induced perturbations in PCO2, HCO3(-), pH, and serum electrolytes are interpreted in the light of sound physiologic principles, even the most complicated mixed disorders may be easily diagnosed. The pathophysiology underlying the simple disturbances is defined and used as a framework for discussion of the mixed metabolic, respiratory acid-base derangements. Formulae are presented which allow one to predict what the appropriate degree of metabolic compensation should be for any primary respiratory disorder, or what the PCO2 should be for any given degree of primary metabolic acidosis or alkalosis. The various clinical settings in which mixed acid-base disorders occur most commonly are discussed.