Microcalorimetry has been used in a series of experiments to comparatively study the toxic effect caused by different diphenol species on microbial activity of Chinese fir soil. Activity of microorganisms on 1.20 g of soil sample was stimulated by 5.0 mg of glucose and 5.0 mg of ammonium sulfate held at 28°C and 35% moisture content. Power-time curves recorded by microcalorimetry demonstrate decreasing amounts of soil microbial activity following increased additions of diphenol. This decrease is likely due to an increase of toxic products formed from degraded diphenol species, adversely affecting the conditions for life in this ecosystem. Among these diphenol species, the toxicity of three diphenols in descending sequence is: hydroquinone, catechol and resorcinol.