In this investigation the base excess of cat foods (sum of alkalogenic compounds calcium, magnesium, sodium and potassium minus sum of acidifying compounds phosphorus, chloride, methionine and cysteine; base excess in mmol/kg dry matter = Ca*2+Mg*2+Na+K-(met(-)+cys)*2-P*2-Cl) as a method of predicting the influence of food on the urinary pH was tested. The base excess and the effect on urinary pH (feeding experiment with 4 to 6 adult cats per trial) was determined in 10 commercial cat foods (3 moist, 3 dry type foods, 4 struvite diets) as well as the influence of several additives (CaCl2, CaCO3, Ca-lactate, CaHPO4, phosphoric acid in 2 doses, NH4Cl, ascorbic acid). After the intake of commercial cat foods the mean urine pH ranged between 6.58 and 7.89, after the intake of struvite diets it ranged between 6.36 and 7.57. The addition of Ca-carbonate or -lactate led to a significant increase of urine pH, Ca-phosphate and ascorbic acid had no effect, while Ca-Chloride, phosphoric acid and ammonium chloride led to a decrease. There was a highly significant correlation between the base excess in the food (x; mmol/kg dry matter) and the mean urine pH (y; y = 6.72 + 0.0021 x; r = 0.96**).