This paper presents an enhanced gas chromatography–mass spectrometry method for the separation of cell gases in polyurethane foam. The novel method was then tested on several polyurethane foams produced at different mixing times, showing successful results. The measurement of gas content in polyurethane foams has been rarely considered in published literature. This parameter, indeed, plays a critical role in the deterioration of polyurethane foam thermal conductivity. This is because of the diffusion of gases which is the main mechanism of foam aging. Hence, an improved gas chromatography–mass spectrometry method was developed to offer simultaneous separation of several types of gas in only one column, using gas chromatography as its main concept. The composition of a sample gas consisting of N₂, O₂, CO₂, and C₅H₁₀ was accurately calculated by measuring the ratio of each peak area on the chromatograms, with argon being used for sampling. This fast and simple method was found to be useful, on one hand for the accurate determination of C₅H₁₀ and CO₂ cell gases used as blowing agents, and on the other hand for N₂ and O₂ air gases that diffuse rapidly from the surrounding environment into foam cells. The effect of mixing time on foam kinetics, cellular structure, foam thermal conductivity, and the overall thermal conductivity of cell gas mixture was also investigated. By complex analysis of foam density, the presence of open cells, cell size, and thermal conductivity of cell gas mixture, the lowest measured value of foam thermal conductivity was explained. The major goal of these experiments was to show the importance of foam cell gas analysis, together with foam structure, which is uniquely done to contribute to the understanding of polyurethane foam thermal conductivity. The thermal conductivity of cell gas mixture is considered as an example of the potential applications of this novel gas chromatography–mass spectrometry method.