To characterize the degree of alteration of the lignin molecule during biodegradation, CuO oxidation has been used in litters and soils. Condensed lignin structures are not attacked by the relatively‐mild alkaline CuO oxidation, for this reason the oxidation degradation with CuO is considered a specific method for the characterization of intact lignin structures in forest litters and soils. In this paper we examine two natural lignins, one from a gymnosperm litter, the other from angiosperm litter and from respective soils under air pollution conditions. The sum of phenolic CuO oxidation products gives an overall pattern of lignin decomposition. The degree of alteration of remnant lignin is described by the acid‐to‐aldehyde ratio (AC:A1) of syringyl and vanillyl units (S:V) for angiosperm lignin. The air pollution conditions do not influenced the degree of lignin degradation.