The influence of the Se evaporation rate during deposition of CuGaSe2 thin films by coevaporation of the elements on Na-containing glass substrates is investigated. A high Se rate is found to avoid the segregation of a Na-containing secondary phase which, in the case of too low Se rates, is deleterious for the performance of the CuGaSe2 films. In addition, the article proposes a model for the phase segregation under low-Se conditions invoking sodium monoselenide as a precursor for the secondary phase. The segregation of the Na-containing compound in CuGaSe2 is further assisted by Ga diffusion from the CuGaSe2 film into the glass substrate. The present results also show that ionic exchange between Na from the glass and Ga from the absorber is active during the growth of Cu(In,Ga)Se-2 alloys on Na-containing glass.