The low temperature gas-phase oxidation of CH3CHO and CzHsCHO have been investigated as an example of heterogeneous-homogeneous reaction. It has been shown that with the increase of S/V (surface/volume) ratio of the reaction vessel the maximum rate of the process increases and the heterogeneous consumption of aldehyde prevails over the homogeneous one. Peroxy radicals have been discovered by EPR method in vessels with different S/V ratios. The concentration of peroxy radicals and the accumulation rate of peracid in the gas phase did not change with the increase of S/V ratio in the same interval. Under the conditions of these experiments excluding the homogeneous propagation of chains the aldehydes oxidized heterogeneously. The peroxy radicals initiate this heterogeneous process which indicate the existence of short chains on the surface. The reaction between the peroxy radicals and aldehyde on the solid surface (SiOz, KCl/Si02) has been shown by EPR, IR-spectroscopy, and mass-spectrometry methods. A conclusion has been drawn that the solid surface takes part in the chain propagation stage of aldehydes oxidation,