The Cooks' kinetic method and tandem-in-space pentaquadrupole QqQqQ mass spectrometry were used to measure primary and secondary kinetic isotope effects (KIEs) in H+ and Cl+ (X+) affinity for a series of A/A' isotopomeric pairs. Gaseous, isotopomeric, and loosely bound dimers [A...chi (+)...A] were formed in combinations in which chi = H+, D+, Cl-35(+) or Cl-37(+) and A/A' = acetonitrile/acetonitrile-d(3), acetonitrile/acetonitrile-N-15, acetonitrile-d(3)/acetonitrile-N-15, acetone/acetone-d(6), acetone/acetone-O-18, acetone-d(6)/acetone-O-18, pyridine/pyridine-d(5), pyridine/pyridine-N-15, pyridine-d(5)/pyridine-N-15, or 3-(Cl-35)chloropyridine/3-(Cl-37) chloropyridine. Under nearly the same experimental conditions, the dimers were mass-selected and then dissociated by low-energy collisions with argon, yielding AX(+) and A'X+ as the fragment ions. KIEs were measured from the changes in ion affinities of the neutrals (AXI) as estimated by the AX(+)/A'X+ abundance ratios. Using [A...H+(D+)... '] and [A...Cl-35(+)(Cl-37(+))...A'] dimers and by comparing their extent of dissociation tinder nearly identical collision-induced dissociation conditions, the kinetic method was also applied, for the first time, to measure primary KIEs of the central ion as well as their influence on secondary KIEs. Becke3LYP/6311++G(2df,2p) calculations were found to provide Delta(Delta ZPE)s for the competitive dissociation reactions that accurately predict the nature (normal or inverse) of the measured KIEs. Copyright (C) 2001 John Wiley & Sons, Ltd.361011401148