The paper deals with the effect of water quality on the pH value of fungicides, insecticides, mineral fertilizers and their mixtures. The fungicides propineb (Antracol WP-70) and mancozeb (Dithane M-70), insecticides pirimiphos-methyl (Actellic-50) and imidacloprid (Confidor 200-SL), several fertilizers (Ferticare I, Ferticare II, Ferticare III and Wuxal Super) and their mixtures were analyzed for pH value under laboratory conditions using a potentiometric pH meter. Measurements were made directly after preparation or mixing with tap and well water and 24 hours later. Tap water exhibited a neutral reaction. A slightly alkaline reaction of well water was most likely due to high ammonium content. The suspensions of Antracol WP-70 exhibited slightly alkaline reactions with both water types during 24 hours. The spray liquids of Dithane M-70 mixed with tap or well water had neutral reaction after preparation and slightly alkaline reaction after 24 hours. The emulsions of Actellic-50 showed neutral reaction with both water types, followed by a pH increase in tap water after 24 hours. The solutions of Confidor 200-SL had a slightly alkaline reaction after mixing and the pH value increased with both water types after 24 hours. It is therefore recommended to apply these insecticides directly after preparation. Mineral fertilizers considerably reduced pH values of the fungicide and insecticide components in double and triple mixtures, especially Ferticare nutrients which had a moderately acid reaction. Wuxal Super had a neutral reaction with both water types. The mixtures with well water increased pH values, which indicates that water pH does affect the pH value of the mixture. Both individual fertilizers and all mixtures (double and triple) with Ferticare had pH values between 2.4 and 6, which allows their active liquids to be stored for 12 to 24 hours. The suspensions (Antracol WP-70, double and triple mixtures), emulsions (Actellic-50 and Actellic-50+Wuxal Super) and solutions (Confidor 200-SL, Wuxal Super and Confidor 200-SL+Wuxal Super) had pH values between 6.7 and 7.9; therefore, the optimum time of their application, especially in case of the mixtures with well water, is immediately after preparation. The suspensions of Dithane M-70 with the insecticides and Ferticare III showed moderately acid reactions with tap water and slightly acid reactions with well water. Since both individual components and their mixtures showed slightly higher pH values with well water than with tap water, it was an indication that water pH may affect the pH value of active liquids.