Experimental and theoretical studies are reported on oscillatory cool flame oxidation of acetaldehyde by oxygen in a CSTR. Several experimental tests are designed to provide information about the reaction mechanism. The concentrations of acetaldehyde, oxygen, methane, methanol, carbon dioxide, water, hydrogen peroxide, and formaldehyde are measured as a function of time with mass spectrometry. Relative amplitude of oscillation, relative phase of oscillation, and the regulation of several species in response to a step perturbation of either acetaldehyde or oxygen inflow are computed from the measurements of concentration. These quantities are calculated for two models of the acetaldehyde reaction. Measurements suggest the eight cited species are chemical "nonessential" variables. (Their concentrations can be individually fixed while other variables oscillate.) Calculations indicate that the "essential" variables in each model are a short-lived chemical species, several radicals, and the internal gas temperature. The measurements of concentration shift regulation of the eight cited species, in response to step perturbations of acetaldehyde or oxygen inflow, and the calculations made with the two models agree except for the responses of carbon dioxide and water to a step perturbation of acetaldehyde inflow. The concentration shift regulation of nine variables for a step perturbation of the bath temperature is inferred from the literature. Calculations of concentration shift regulation for the first model show agreement for three variables and disagreement for four variables; calculations for the second model show agreement for six variables and disagreement for two variables. Measurements and calculations of the relative phases of oscillation between acetaldehyde, oxygen, methane, methanol, internal temperature, and hydroxyl radical agree. Both models are "category 2" oscillators. The "negative feedback" species is peracetic acid in the first model and methyl hydroperoxide in the second. The "autocatalytic" species in both models are groups of radical intermediates.