Negative valve overlap (NVO) is an operating mode that enables efficient, low-temperature gasoline combustion in automotive engines. In addition to retaining a large fraction of residuals, NVO operation also enables partial fuel injection during the recompression period as a means of enhancing and controlling main combustion. Thermal effects of NVO fueling on main combustion are well understood, but chemical effects of the products of NVO reactions remain uncertain. To address this topic, we have fabricated a dump valve that extracts a large fraction of cylinder charge at intake valve closing (IVC), yielding a representative sample of NVO products mixed with intake air. Sample composition is determined by gas chromatography. Results from a sweep of NVO start-of-injection (SOI) timings show that concentrations of the reactive species acetylene and hydrogen rise to several hundred parts-per-million as NVO SOI is retarded toward top center of NVO. Since experiments have previously demonstrated that low concentrations of acetylene seeded into the intake flow advance combustion phasing, the current results support the conclusion that NVO fueling can chemically enhance main combustion. This conclusion is further strengthened by a one-dimensional chemical kinetics simulation of main combustion that uses measured compositions as initial conditions at IVC. Comparing early and late NVO SOI, the model predicts that the hydrogen and acetylene produced by late NVO injection significantly advance the phasing of main combustion.