The relation between modification intensity and decay resistance of modified wood is investigated based on a compilation of experimental data from literature for six different modification techniques. The purpose is to expand our knowledge on the mechanism of wood modification, in particular how decay resistance is achieved. Decay resistance of modified wood appears to be related to reduction in maximum moisture capacity of the cell wall. The analysis indicates that decay cannot progress below 25% moisture content. The moisture exclusion efficiency (MEE) and anti-swelling efficiency (ASE) are both discussed as means of quantifying modification efficacy. Both MEE and ASE have advantages and disadvantages, but MEE seems to provide a threshold for decay resistance unaffected by type of modification. However, MEE cannot be determined at water saturation and the use of it as a measure for modification efficacy therefore relies on the assumption that MEE is more or less similar at saturation and below.