The conditional stability constants for various modifications of molybdenum heteropoly anions (HPAs) having Keggin structure were calculated using the results obtained when constructing the molar ratio curves. A comparative study of the stability of the main analytical forms of HPAs, which were used to determine phosphorus(V) and gallium(III), was carried out including oxidized, metal-substituted, reduced and oxidized forms, as well as those obtained in non-ionic surfactant medium. Based on the results of the study, molybdenum HPAs of phosphorus(V) can be arranged in the next row according to the values of conditional stability constant: PMo12O40 3-(in a non-ionic surfactant medium) (β = (1.60.6)•10 40) > H4РBi 3+ 2Mo4 V Mo7 VІ O40 6 (β = (5.61.6)•10 39) > H4РMo4 V Mo8 VІ O40 3-((β = 4.90.4)•10 37) > PMo12O40 3-(β = 10 36). The members of the series with n = 2-4 were the most stable in the series of gallium molybdotungsten HPAs. The stability of various forms of molybdenum HPAs of phosphorus(V) is not very different and is determined by the stability of the entire skeleton of Keggin heteropoly anion. When assessing overall stability of reduced forms of HPAs, it is important to consider their kinetic stability.