This dataset contains the temperature and relative humidity in a bypass to the interstitial inlet, measured during the MOSAiC expedition from October 2019 to September 2020. The measurements were performed in the bypass of the interstitial inlet of the Swiss container on the D-deck of Research Vessel Polarstern, using a commercial hygrometer model HC2 (Rotronic AG, Bassersdorf, Switzerland). The interstitial inlet had a flow of > 17 L/min and was equipped with a 1 µm cyclone, for sampling interstitial particles only, as opposed to the total inlet (flow of > 15 L/min) which sampled all particles and hydrometeors up to 40 µm in diameter. The total and interstitial inlets were located 3 meters apart and connected to a valve that switched hourly between the two inlets for a targeted set of instruments. The complete instrumental setup inside the Swiss container is presented in Heutte et al. (Submitted), Dada et al. (2022), and Beck et al. (2022). Inside both inlets, the temperature was kept constant around 20 °C and the relative humidity was maintained below 40 %, with a heating system following Global Atmosphere Watch (GAW) standards for aerosol sampling (WMO, 2016). This dataset contains measurements of temperature and relative humidity inside the bypass to the interstitial inlet, averaged to 10 min time resolution (native resolution was 5 min), along with the position of the switching valve (1 = total inlet, 0 = interstitial inlet).