The photochemistry of oxygen delignified softwood kraft pulp was studied. The effect of pH and temperature of the hydrogen peroxide treatment under alkaline and acidic conditions was investigated. The diffuse-reflectance laser-flash photolysis method and ground state diffuse reflectance spectroscopy were employed for the study. Results suggested that, under acidic conditions, hydrogen peroxide followed by an alkaline extraction stage was useful as a delignifying agent in pulp and paper industry.