The present invention provides a method and gaseous composition for the in situ bioremediation of soil and groundwater contaminated with organic compounds, including halogenated hydrocarbons. The gaseous composition, which readily permeates a subsurface region, comprises hydrogen (H.sub.2) and one or more volatile phosphates, such as triethylphosphate (TEP) and tributylphosphate (TBP). The volatile phosphates serve as nutrients that stimulate the growth and activity of indigenous microbes that are capable of degrading the contaminants. The addition of hydrogen facilitates the direct reductive dehalogenation of highly halogenated contaminants. The gaseous composition may optionally contain one or more of a volatile alkane and nitrous oxide as additional supports for microbial growth, and carbon dioxide to lower the pH of remediation sites that are highly alkaline.