The diffusion of Cr in GaAs was investigated under both in‐diffusion and out‐diffusion conditions. In‐diffusion sources were CrAs(s)‐GaAs(s)‐Ga(l), three phase mixtures which, at a given temperature, provide an invariant chemical potential of the components. Unexpectedly deep penetration was observed and complex (two‐branch) concentration profiles were often observed. The profiles were successfully modeled using an interstitial/substitutional/dissociative model. Different regimes of diffusion conditions gave rise to fast and slow effective diffusion coefficients. Subtle effects of doping and preannealing under arsenic overpressures confirmed the proposed model and served as a probe to study these subtle differences. Out‐diffusion experiments led to slow effective diffusion coefficients, consistent with the diffusion model.