The structures and energetics of nine valence isomers of phosphinine, (CH)(5)P, have been investigated by ab initio (HF, MP2 and CCSD(T)) and hybrid density functional (B3LYP) methods. The relative stability ordering of the (CH)(5)P isomers is similar to those of (CH)(6). Strain energies are evaluated for all the non-planar isomers based on the sum of standard bond strengths, taking the planar resonance stabilized isomer, phosphinine as the reference. Lower magnitudes of the frequencies corresponding to the first few normal modes compared to their benzene isomers account for smoother isomerization reactions among them.