Abstract: The particle size distribution, total and exchangeable Mg, and mineralogical compositions were determined on eight well-drained, noncultivated subsoils from Pennsylvania. No correlation was found between the clay content and total Mg (r = .29), or between the clay content and exchangeable Mg (r = .35). Serpentine, talc, and hypersthene were found in the very fine sand and silt fractions of soils having relatively high exchangeable Mg. Mica and 14-Å clay minerals were the only Mg-bearing minerals noted in the same fractions of soils having relatively low exchangeable Mg. Of the Mg-bearing clay minerals found in the clay fractions (smectite, vermiculite, chlorite, illite, and interstratified chlorite/vermiculite), only smectite decreased as the exchangeable Mg in the soils decreased. Two distinctly different distribution patterns of Mg were found for soils having relatively high and low exchangeable Mg. The former soils showed a decreasing Mg content as the particle size decreased, and the latter soils showed the opposite. Exchangeable Mg correlated significantly with the amount of Mg in whole soil, sand, and silt, but not with the amount of Mg in the clay, an indication that sand and silt but not clay were the important sources of exchangeable Mg in these soils.