Methods based on UV-visible diffuse reflectance spectroscopy were used to study the physiological aspects of lignin-peroxidase biosynthesis by Phanerochaete chrysosporium. Here we introduce the use of cytochrome aa3 as an indicator of active fungal biomass and of its redox state to calculate the oxygen mass transport coefficient between the growth medium and the fungal cell interior. When lignin peroxidase biosynthesis was enhanced by the addition of Tween 80 or Tween 20 to the growth medium, a higher proportion of reduced cytochrome aa3 and a higher oxygen diffusion barrier were observed compared with control cultures. In cultures supplemented with Tween 80 or Tween 20, a higher oxygen mass transport coefficient between the growth medium and the interior of the fungal cell was also found. The beginning of the lignin peroxidase activity in these cultures was found to coincide with a temporary cessation in the dry biomass increase and a reduction in the relative active-biomass concentration. During the lignin peroxidase activity, a decrease in the intracellular pH and an increase in the growth medium pH were determined in cultures supplemented with Tween 80.