Capacitive relative humidity sensors, widely used in industrial and environmental applications, should be interchangeable and ideally have parametrizable response to relative humidity changes. While the effects of temperatures in the range between 0 degrees C and 50 degrees C are negligible, this is not the case for negative values. Therefore, this effect must be taken into consideration when parameterizing the output of the capacitive humidity sensors for temperatures below the freezing point of water. In this paper, we present our investigations on the temperature effect for one type of such sensors when using it at subzero temperatures. The experiments were conducted at temperatures of -10 degrees C, -20 degrees C and -30 degrees C. Additionally, we have tested the sensor performance at relative humidity levels low enough for most sensors not exhibit their best performances. Direct and inverse calibration methods were used and the uncertainty analysis of the devices under test is discussed.