The American Petroleum Institute (API) funded efforts to develop three methods for measuring petroleum hydrocarbons in soil. The methods have undergone extensive single laboratory validation studies. This work has been presented. In June of 1992, API sponsored an interlaboratory study to collect additional data. A total of 15 laboratories from both the commercial and government sectors participated in the study. The study was conducted in accordance with ASTM D 2777-86 and involved Youden pairs at three concentrations for each method. Stable test materials were prepared as spiked soil samples. The spike concentrations were verified. Following removal of outlying results, the precision and accuracy were calculated using the techniques in ASTM D 2777 and Youden's technique of calculating intralaboratory (single analyst) precision. Our manuscript will review the analytical methods, describe the study design, including the preparation of stable test materials, and present the results from our study.