Loss of weight on ignition (LOI) at 360°C for 2 h was compared with organic matter (OM) determined by "Walkley-Black" titration in 356 Wisconsin soils. The effects of sample size, number of samples heated at one time, and time of heating were studied using these soils. The number of samples heated at one time in a muffle furnace did not affect LOI. For an organic soil (34.2% OM), beaker size (20-or 50-mL) was unimportant, but LOI increased as sample size decreased. Sample size was not significant for a mineral soil (3.62% OM). Time of heating was significant for both soils.A coefficient of determination (R 2 ) of 0.97 was obtained for the linear regression between OM and LOI of the entire group, which had OM ranging from 0.1 to 54.4%. When only samples containing less than 10% OM were considered (n=316), the regression equation was: OM = 0.973 LOI -0.33 (R 2 = 0.90).