Recent task group activities in support of ASTM Committee E37 efforts to characterize volatile chemicals with thermal analysis have involved the use of precision pinholes in sealed specimen containers. During the laboratory testing in support of these efforts, occasional artifacts were encountered. In particular, the development of ASTM Test Method for Determination of Vapor Pressure by Thermal Analysis (E 1782) showed some sporadic exothermic activity near the onset to boiling at test pressures significantly greater than ambient. This exothermic activity is associated with partial oxidation of the specimen resulting from residual oxygen as a contaminant in the vapor, consisting otherwise of a self-generated atmosphere within the specimen container. This oxidation occurs only when the working pressure is significantly greater than ambient and the boiling temperature of the specimen approaches 200°C. The source of the oxygen contaminant is laboratory air trapped with the specimen during encapsulation and laboratory air from the DSC sample chamber partitioned into the specimen container during equilibration of the applied test pressure across the pinhole. This contamination reaction can be avoided either by initially drawing vacuum and subsequently pressurizing the sample chamber of the DSC with inert gas to the desired pressure, or by flushing the sample chamber with inert gas in a series of nominally eight successive pressurize-vent-repressurize repetitions to the desired pressure with inert gas before initiating the heating program.