Compliance matrices of protonated CO and N-2 are calculated using coupled cluster methods and basis sets of quadruple zeta quality. Diagonal elements of the compliance matrices are used as unique bond strength descriptors. Going from CO (0.052 Angstrom/mdyn) to CO-H+ the C-O bond is weakened (0.062 Angstrom/mdyn), while the C-O bond in H-CO+ is getting stronger (0.045 Angstrom/mdyn). After protonation, the N-N bond strength is getting stronger (from 0.043 to 0.042 Angstrom/mdyn), too. The invariance of compliance matrix elements C-ij under completion of (x(i),x(j)) to a complete set (...,x(i),...,x(j),...) of internal coordinates is demonstrated. (C) 2003 American Institute of Physics.