The potential energy surfaces (PESs) for the primary and secondary dissociations of the phenylarsane molecular ion (1a) were determined from the quantum chemical calculations using the G3(MP2)//B3LYP method. Several pathways for the loss of $H{cdot}$ were determined and occurred though rearrangements as well as through direct bond cleavages. The kinetic analysis based on the PES for the primary dissociation showed that the loss of $H_2$ was more favored than the loss of $H{cdot}$, but the $H{cdot}$. loss competed with the $H_2$ loss at high energies. The bicyclic isomer, 7-arsa-norcaradiene radical cation, was formed through the 1,2 shift of an $alpha$-H of 1a and played an important role as an intermediate for the further rearrangements in the loss of $H{cdot}$ and the losses of $As{cdot}$ and AsH. The reaction pathways for the formation of the major products in the secondary dissociations of $[M-H]^+$ and $[M-H_2]^{+cdot}$. were examined. The theoretical prediction explained the previous experimental results for the dissociation at high energies but not the dissociation at low energies.