Clinically used infusion solutions are complex aqueous mixtures composed of a variety of different salts, acids, and non-electrolytes, and are characterized by composition, initial pH, and titratable acidity (TA). By rigorous treatment as a multi-composed acid-base system, pH and TA were calculated from proton dissociation equilibria, mass balance equations and electroneutrality condition. Nine were arbitrarily chosen as model solutions in which pH and TA were experimentally determined. From composition, and based on a set of apparent acidity constants (pK values) at 37 degrees C and total ionic strength 0.155 mol/l, pH was calculated by iteration. In the model solutions, measured pH was in the range 5-7, and the difference between calculated and experimental values was+/-0.064. Good agreement was also found between calculated and measured TA: in the range 0-55 mmol/l the mean difference was+/-2.07 mmol/l, if calculated from measured pH, and +/-0.95 mmol/l, if calculated from calculated pH.