In order to enhance the understanding of surface processes involved in MOCVD, the desorption of As and P from the (001) surface of binary and ternary III-V semiconductors was studied by in situ ellipsometry. The variations of ellipsometric parameters as a function of time give the group V desorption rate constant and Arrhenius laws were determined for all the materials. The evolution of activation energies suggests that these are influenced by the surface bond strengths. These kinetic data are essential for the control of heterostructures growth and the construction of numerical model devoted to the growth of InGaAsP. The study also suggests some specific surface reconstructions, especially in the case of the GaAs As-rich surface which probably adopts a c (4 x 4)-like structure for T greater than or equal to 625 degreesC and P-TBAs = 2 Pa and would present an As excess over the c (4 x 4) structure for T less than or equal to 600 degreesC.