The Residence Time Distribution (RTD) technique combined with mass spectroscopic Residual Gas Analysis (RGA) is employed, for the first time, to study the nature of nonideal gas flow in atmospheric thermal silicon oxidation reactors. The RTD results are interpreted in terms of various dispersion models. Higher flow rates improve plug flow conditions by reducing the Dispersion Number, D. In cases where baffles are not used upstream of the wafers, the Finite Length System (FLS) model agrees most closely with the experimental data due the presence of macroscopic variations in flow caused by the entering gas jet. When baffles are present, the Semi-Infinite Length System (SILS) model is shown to apply. Increasing the number of baffles and the longitudinal location results in lower D. The trends are explained theoretically by determining the extent of natural convection which has been shown to be the major obstacle towards achieving plug flow conditions in thermal silicon oxidation reactors.