Lignin concentration in the S3 layer of the tracheid wall of radiata pine was determined by making porosity measurements of hydrolysed cell walls using interference microscopy, with appropriate corrections for swelling. The mean S3 lignin concentration was approximately 53% v/v. The degree of swelling of different wall layers as a result of hydrolysis was proportional to the lignin concentration in that layer with values of 1.645 for the S2 layer, 1.357 for the S3 layer and 1.053 for the compound middle lamella.