Laboratory experiments were conducted to investigate how both mineral soil and moisture content affect the smoldering combustion in forest duff. Peat was used to represent the fermentation and humus horizons (Oe and Oa soil horizons) of a coniferous forest floor nominally called duff. Although moisture content is traditionally considered to be a major factor limiting the combustion of duff, inorganic content from mineral soil also limits combustion. Results show that the peat moss selected sustained smoldering up to the range 93 to 103 % moisture content in its natural state of 3.7% inorganic content (both relative to the dry organic mass). Adding inorganic material in the form of finely divided mineral soil (clay to silty clay) depressed the moisture limit that previously permitted smoldering. Smoldering is not sustained when the inorganic to organic ratio reaches 4.3 (430%) in the absence of moisture. Between these two extremes, there is an approximate linear decrease in the moisture limit with increasing inorganic content. A comparison with the limits of smoldering for duff (fermentation) samples collected from under an inland Douglas-fir (Pseudotsuga menziesii var. glauca (Beissin.) Franco) canopy shows a similar dependence on moisture and inorganic content.