Microgram-scale reactors combined with gas chromatography (GC) coupled to mass spectrometry (MS) or flame ionization detection (FID) are used widely in pyrolysis research. Whether these devices meet the expected fast heating rates and short vapor residence times of fast pyrolysis have not been verified. In this study, experiments and simulations are used to investigate heat and mass transfer in a furnace-based micropyrolyzer. Surprisingly, heating rates obtained from the temperature history of sample cups in the reactor were modest compared to the greater than 1000 K s−1 heating rates sometimes assumed for such reactors. The heating rate at 773 K, employed commonly in fast pyrolysis, was only 180 K s−1. The highest rate observed was 494 K s−1 at a furnace temperature of 1268 K, which is well above typical pyrolysis temperatures. The mass transfer of volatilized samples was studied using both an optically accessible furnace and computational fluid dynamics. The standard sample cups used with these micropyrolyzers impede the escape of vapors. The use of shallow perforated cups overcame this mass transfer limitation to lead to levoglucosan yields ≈10 % higher than usually reported for the pyrolysis of cellulose.