Formation equilibria of molybdate ion (MoO42-) complex with cytosine in pH=5.8 have been investigated by spectrophotometric measurements in aqueous solution at 0.15 mol lit-1 ionic strength (NaClO4), and 25±0.1oC. In this condition the 1:1 complex has the formula MoO4L2-, where L represents the base of DNA. The stability constants of the complexes formed and their stoichiometries are given and interpreted. The logarithms of the cumulative stability constant bxyz of the complex: [(H)x(oxometal)y( cytosine )z], is log b211=17.321.