Despite recent advances, little has been experimentally established regarding the detailed chemical processes that occur during biomass pyrolysis reactions. We developed a new technique that allows for the monitoring of each molecular product from fast pyrolysis with ∼0.2 s temporal resolution. This was achieved by directly coupling a micropyrolyzer with a time-of-flight mass spectrometer via a soft ionization. Molecular products studied were produced in thin-film pyrolysis of a series of glucose-based carbohydrates. Unprecedented details of the pyrolysis reaction process were revealed, including the timescale of molecular product formation and the existence of metastable intermediates. Small carbohydrates are completely pyrolyzed within one second and as short as one-half second for glucose pyrolysis. Individual time profiles could be extracted and examined for each molecular product. Additionally, the effect of sample dimensions on the pyrolysis of cellulose and α-cyclodextrin, as both thin films and particles, was studied. A surprising time delay of one second is observed for the thin-film pyrolysis of cellulose and α-cyclodextrin, which is attributed to the transition to the molten phase. When a large amount of cellulose or α-cyclodextrin is pyrolyzed, random fluctuations of temporal profiles are observed and explained as coming from aerosol ejections. This is well correlated with the high abundance of non-volatile products such as cellobiosan that cannot be detected in typical GC–MS or pyrolysis GC–MS analysis.