A hybrid tandem instrument of BEqQ geometry was used to determine high-energy decomposition of protonated peptides, such as side-chain fragmentation yielding d n and w n ions. The transmission through both E and Q of such product ions, formed in the second field-free region, permits improved mass resolution and confident mass assignment. The experimental technique may involve synchronous scanning of E and Q, or, for the purpose of identification of specific products, limited-range scanning of either E or Q with the other analyzer fixed. These techniques are not equivalent, with respect to product ion transmission, to the double focusing of product ions achieved with four-sector instruments but nevertheless represent a critical improvement over conventional mass-analyzed ion kinetic energy spectrometry analyses. Fragmentation of protonated peptides occurring in the second field-free region inside and outside the collision cell were distinguished by floating the collision cell above ground potential. Mass filtering using Q confirmed the mass assignments. The data indicate that product ions resulting from spontaneous decomposition are in some instances quantitatively more significant than those resulting from high-energy collisional activation. Furthermore, the differentiation of the products of low- and high-energy processes should facilitate spectral interpretation.