The two-dimensional (2D)-three-dimensional (3D) transition has been studied for InAs and InP layers grown on (001) GaAs substrates by metalorganic vapor pressure epitaxy at different growth temperatures and growth rates. The combined use of scanning force microscopy and of Rutherford backscattering spectrometry allowed the determination of the surface morphology of the samples and of their equivalent layer thickness with a precision better than 0.1 monolayers (ML). The critical thickness for the 2D-3D transition has been found to occur after a deposition of (1.2+/-0.1) ML for InAs/GaAs and after a deposition of (2.2+/-0.2) ML for InP/GaAs, independently of the temperature and of the growth rate. For coverages slightly exceeding 1 ML, for InAs, and 2 ML, for InP, the surface of the samples appears partially covered by small bi-dimensional structures (platelets). The dimensions of the platelets appear to be independent of the system under study and consequently of the misfit. Moreover, their distribution on the surface appears to be strongly different from that of the islands after the 3D transition so that they cannot strictly be viewed as island precursors, as previously suggested.