In this research work five different diets were tested in relation to their efficiency and suitability for therapy and prophylaxis of struvite and calcium oxalate uroliths. The dry diets included two commercial diets (Hill’s s/d, RC Urinary S/O); two test diets for therapy or prophylaxis of struvite urinary stones (CURD, URP); and a diet for adult cats as a control (ACC). The trial, of crossover design, proceeded in five replicates with ten cats from the Institute of Animal Nutrition of the University of Veterinary Medicine, Vienna.The study included measurements of volume, pH and specific gravity of urine, a struvite dissolution trial and RSS-analysis. In each replicate the dissolution trial (one gram of struvite crystals were mixed with 20 ml of filtered urine) was conducted for a period of two and five days, with three respective samples for each cat and period. Incubation in a water bath was realized at a temperature of 38 ºC with a shaking frequency of 80/minute. After filtration, drying and weighing, the quantity of dissolved crystals was determined.From all the researched diets, Hill’s s/d was the most effective diet for struvite dissolution according to the statistic evaluation, with a significant lower pH-value, the lowest struvite-RSS and also the highest solubility of struvite crystals. According to the results of the research, both test diets with high protein content (CURD, URP) comply with the requirements for therapy and/or prophylaxis of struvite and calcium oxalate stones. The diet for dissolution of struvite stones (CURD) was comparable with both commercial diets. The preventive diet (URP) achieved advantageous values for recurrence prophylaxis. The results of the pH- and RSS-research were confirmed by the in vitrosolubility of struvite crystals. All diets except ACC should dissolve struvite stones and all diets according to the determined RSS-values should have a prophylactic effect against calcium oxalate stones.