The effects of blockiness and compositional/sequential polydispersity on the order–disorder transition of random block copolymers are studied using a segment model of random block copolymers. Specifically, the copolymers are randomly assembled from a collection of equal‐length “segments,” while the characteristics of the segments are chosen according to the nature of the polydispersity. The phase behavior of the model system is examined using the random phase approximation. The critical points of the system are calculated over a range of blockiness and polydispersity for a variety of segment models. It is observed that the critical value of the Flory–Huggins parameter χ, above which the homogeneous phase becomes unstable, is inversely proportional to the blockiness. Furthermore, it is found that sequential and compositional dispersity decreases the critical value of χ, but that the effect of sequential dispersity is much weaker. At sufficiently large compositional dispersity, the random block copolymers can undergo macrophase separation. In this region of phase space, the critical point is entirely determined by the compositional polydispersity.