This work aims to determine the major coke precursors in n-heptane reforming over a commercial Pt-Re/Al2O3 catalyst. We employ both a multioutlet fixed-bed reactor and a new microbalance that measures coke-on-catalyst by monitoring the natural vibrational frequency of a tapered element that is pinned at one end and has catalyst at its free end. This microbalance does not have the gas bypass problem common to conventional gravimetric microbalances. We show that five-carbon ringed naphthenes (C5N) are the only reforming reaction participant species lump whose concentration rises whenever the rate of catalyst activity loss increases in response to changes in temperature, pressure, or space velocity. A more quantitative treatment shows that the C5N concentration profile along the fixed-bed reactor tracks both the coke profile and the profile of a measure of the local catalyst deactivation rate. Further detailed microbalance studies with different hydrocarbon feeds show that the coking rate is first order in C5N and is almost independent of toluene concentration. The C5N formed in n-heptane reforming quantitatively accounts for most of the coke produced. All experiments pointed to C5N as the major coke precursor in n-heptane reforming.