Helium is often measured in exploration programs, either in interstitial soil gases collected by hollow probe or in gases desorbed from pore spaces of soil samples. However, measured helium concentrations are affected by various parameters that muts be considered when interpreting data, including the variation in concentration of other soil gases. The effects of moisture and carbon dioxide in the soil are particularly important. This study compares helium concentrations with soil moisture and carbon dioxide contents of soil and soil gas samples collected in several areas. Where soil moisture was below about 4% by weight, helium concentrations in soil gases collected by hollow probe correlated well with helium concentrations desorbed from soil at the same site. Measured helium concentrations were significantly reduced, however, in areas where soil moisture was greater than 10% and were below the concentration of helium in air in areas where soil moisture exceeded 18%. Carbon dioxide content of the samples also affects helium concentrations. Helium concentrations were significantly reduced in samples where the carbon dioxide content exceeded 1% by volume of the gas sample. ¿ American Geophysical Union 1987