The study of degradation and growth patterns of fungi in modified wood may increase the understanding of their mode of action and may lead to more accurate service-life predictions. The aim of this paper was to study the degradation and growth patterns of brown rot fungi in modified wood and to measure moisture content (MC), structural integrity and the acetyl content by frequent monitoring over 300 days. Mass loss (ML) in the modified wood materials increased slowly up to 3% for 50–100 days after which it flattened out and remained constant during the remainder of the test. Structural integrity and acetyl content were maintained in the modified wood materials and MC was lower compared to untreated wood throughout the decay test. ML results of untreated wood indicate that fungi in solid wood go through distinct phases; the degradation patterns in the modified wood materials were more difficult to interpret.