Sorptive interactions with indoor surfaces strongly influence indoor exposure to organic pollutants. Adsorption itself may be influenced by indoor levels of common indoor gases such as CO₂, NH₃, and H ₂O. We quantified sorption characteristics of trimethylamine (TMA) on carpet and painted wallboard, while challenging the surface with gas-phase CO₂, NH₃ and H₂O. We show that the capacity of the carpet to sorb TMA, doubles when the CO₂ mixing ratio is increased from 0 to 1000 ppm CO₂ at 90% relative humidity. In contrast, NH₃ decreases the surface capacity of both carpet and latex paint. Sorption of TMA to these indoor materials is primarily caused by interactions at one or more interfaces. Dissolution of TMA and aqueous acid-base chemistry appear to also contribute to the overall sorptive capacity of carpet at high relative humidity. The reduction in the distribution coefficient, k _ε, in the presence of NH₃ is explained by competition between TMA and NH₃ molecules for sites on the substrates at low-to-medium relative humidity conditions.