A graph theoretical formulation of the PPP method is presented. A weighted adjacency matrix of the PPP graph is given, wherein the off-diagonal elements are the bond orders. The automorphism group of the PPP graph is defined and shown to be isomorphic with the permutational subgroup of the permutation-inversion group of the molecule. It is demonstrated that the characteristic polynomial of the adjacency matrix of the PPP bond graph is invariant in every SCF iteration. It is shown that the PPP spectra discriminate isospectral graphs.