Much of human exposure to nitrogen oxides (NOx) of ambient origin occurs indoors. Reactions with materials inside building envelopes are expected to influence the amount of ambient NOx that infiltrates indoors. However, envelope penetration factors for ambient NOx constituents have never been measured. Here, we develop and apply methods to measure the penetration factor and indoor loss rates for ambient NOx constituents using time-resolved measurements in an unoccupied apartment unit. Multiple test methods and parameter estimation approaches were tested, including natural and artificial indoor NOx elevation with and without accounting for indoor oxidation reactions. Twelve of 16 tests yielded successful estimates of penetration factors and indoor loss rates. The penetration factor for NO was confirmed to be ∼1 and the mean (±s.d.) NO2 penetration factor was 0.72 ± 0.06 with a mean relative uncertainty of ∼15%. The mean (±s.d.) indoor NO2 loss rate was 0.27 ± 0.12 h-1, ranging 0.06-0.47 h-1, with strong correlations with indoor relative and absolute humidity. Indoor NO loss rates were strongly correlated with the estimated ozone concentration in infiltrating air. Results suggest that envelope penetration factors and loss rates for NOx constituents can be reasonably estimated across a wide range of conditions using these approaches.