Threshold photoelectron spectra of Ar and N2 are studied by the technique of attachment of the threshold electron to SF6 followed by detection of the SF6−. Studies on Ar provide a measure of the rejection ratio for nonthreshold electrons of this technique. This ratio is compared to that for two other types of electron detectors. Collisional ionization of high Rydberg-state electrons by SF6 is also observed and discussed. In N2 direct ionization to vibrational levels of the X 2Σg+, A 2Πu, and B 2Σu+ states of N2+ are clearly resolved. Moreover, resonant autoionizations are found to greatly enhance the population of the vibrational levels of the X state and the v′=2 level of the A state over that expected from direct photoionization alone. Extra peaks lying between the vibrational levels are detected which correspond to autoionization into high-J levels of the N2+ ion. From intensity considerations a rotational propensity rule is given which states that an autoionizing N2 state prefers to decay to N2+ states with a minimum change in rotational angular momentum. Normalization of the threshold spectra to the absolute cross section scale is also discussed.