Iron alloys containing about 6.5 weight percent silicon are known to respond to heat treatment in a magnetic field. Since the degree of response has been observed to vary with sheet thickness, a study was made to better define this variation.Two different alloy compositions were cast in cylindrical molds. The ingots were drilled to form hollow cylinders which were sliced and polished to rings of final thicknesses of from 0.150‐in. to 0.0025‐in.dc hysteresis loops were measured on samples as annealed 2 hours at 1000°C in dry hydrogen, and as subsequently cooled at 40°C/hr from 900°C in a circumferential magnetic field of 10 oe.The differences in coercive force and maximum permeability between samples field annealed and not field annealed are thickness dependent, and are greater for thicker samples.The effects of thickness on coercive force are lessened by electropolishing, which greatly reduces the slope of theHcvs(1/d)‐curve for both the field‐annealed and not field‐annealed states. The maximum permeability is not found to be affected by electropolishing.