It is experimentally demonstrated that under certain growth conditions, the deposition time of intentionally doped single-crystal GaAs mono-epitaxial layers grown by Metallic-Organo-Chemical Vapor Deposition (MOCVD) on (100) GaAs substrates strongly influences the residual impurities content. Five samples, each grown with a different deposition period but under the same growth conditions and sequence were investigated. The residual impurities in the upper part of the epilayer (0.4μm to 1μm below the layer surface) were determined using C-V measurements. The results show that in this range of depth: (1) the residual impurity concentration is lower in layers deposited during longer periods, and higher in layers deposited during shorter periods; (2) the distribution profile of these impurities can be approximated as linear; (3) the impurity gradient is larger for layers deposited during shorter periods than in layers deposited during longer periods. The above relations between residual impurities and growth time may be important in developing procedures for GaAs devices fabricated by the MOCVD method.