The evolution of solitary waves is studied using the KP equations derived by employing the reductive perturbation technique. It is revealed that the relativistic effect plays a role in the formation of double layers in multi-temperature electron plasmas. Because of the singularity in soliton propagation introduced by the zeros in the nonlinear coefficients of the KP wave equations. Owing to the appearance of zeros in the nonlinearity, there might be a continuous modification of the KP equation. The relevant properties of compressive and rarefactive solitons as well as shock-like soliton are considered. It is possible that the layers of various solitary waves might be observed in plasmas.