We present an in situ study of P desorption and adsorption on the (001) InP surface using surface photoabsorption (SPA). The SPA spectra show three peaks at 2.1, 2.9, and 3.1 eV. Here, based on previous studies, we have identified them as a P dimer related and two surface In dimer related peaks, respectively. The initial sharp drop and the slow recovery of the SPA signal at 430 nm after PH3 is switched on implies that at least two steps are involved in the P desorption process. We also observed a structure at 3.1 eV in the spectrum of the In-stabilized surface. We have determined from temperature dependent desorption measurements that the activation energy for P desorption from this surface is 3.36 eV.