From in-situ ellipsometric measurements during condensation of As4 molecular beam on GaAs(001) substrate the arsenic sticking coefficient was determined. For the substrate with Ga-rich surface reconstruction at 60C the sticking coefficient was found to be 7x10-3, whereas for the surface of amorphous arsenic this coefficient was only 8x10-4. Kinetic studies of As sublimation were also performed and activation energy for the process was obtained. For amorphous As films grown from As2 beam the activation energy was found to be DeltaE=1.84+-0.03eV, which is 0.39eV higher than the enthalpy of solid-vapor phase transition. Temperature dependence of evaporation coefficient for amorphous As was determined in the 230-290C range.