The purpose of this study is to construct a method to predict vehicle cabin VOC (volatile organic compounds) concentration. Several methods have been used previously to evaluate VOC emission from interior parts and materials (e.g., sampling bag method, 1 m3 chamber method). However, measurement conditions vary depending on the method used, making it difficult to predict vehicle cabin VOC concentration from the VOC values evaluated for component parts. In this paper, we focused on measurement of toluene concentration using the bag method and investigated the relationship between VOC emissions and measurement conditions. We assumed that the amount of VOC contained in the parts (RA) and the adsorptive capacity of the parts (K) can describe the VOC amount obtained (RG) when the VOC concentration in the bag reaches equilibrium. We developed a novel method incorporating a labeled compound to predict RA and K. The K value is thought to exhibit additivity with respect to sample amount and vary in inverse proportion to the amount of nitrogen gas present. In this study, we attempted to predict RA and K values of two kinds of sample (TP-B and TP-C) using this novel method. Furthermore, we estimated the value of RG in a bag containing one piece of TP-B and four pieces of TP-C. The estimated value of RG agreed well with that determined using the bag method.