Small environmental test chambers may present wall adsorption for volatile organic compounds (VOC), which must be known for correct emission or adsorption rate determinations on indoor materials. Experiments in a 0.28 m³ stainless steel chamber, in the dynamic (air flow through the chamber) and static mode (no flow), with n-decane and n-dodecane are described. Further experiments with a 0.45 m³ glass chamber and more compounds are reported. The concentration versus time data were used as input for a new mathematical model with two sink compartments (“two-sink” model), which is described. The model provides a much better regression of data points than the previous “sink” model. The two sinks show greatly different rate constants. The model provides a reasonable description of the mass balance in adsorption-desorption experiments.