Abstract: The invention pertains to an immunoassay method for determining the presence of volatile organic compounds in aqueous, soil and air samples by simultaneously collecting and testing a sample volume suspected to contain such organic compounds. As a major problem in the assay of such materials is their rapid evaporation, the present immunoassay is specifically designed to eliminate or minimize the evaporation of the volatile organic analyte during sample handling as well as during the assay process itself. The immunoassay method is based on an assay vessel which has a lower portion, in which the immunoassay actually takes place, and an inert upper portion, which can hold a sufficiently large volume of sample to prevent or minimize evaporation of the organic compound from the smaller volume in the lower portion.