Hybrid System-in-Foil (HySiF) is one of the emerging branches of flexible electronics in which ultra-thin silicon chips are integrated with flexible sensors in polymeric foils (Elsobky et al. 2018; Alavi et al. 2018). Intensive attention was given to the implementation of flexible environmental sensing platforms for logistics and food packaging (Cartasegna et al. 2011; Liu et al. 2016). The aim of this work is the implementation of a sensor system demonstrator using HySiF components, namely an ultra-thin microcontroller chip in addition to an on-chip temperature and an on-foil humidity sensors. The measurement concept for the relative humidity sensor is measuring the capacitance difference between an off-chip (on the foil substrate) humidity dependent sensor capacitor, and another humidity independent reference capacitor. The electrical readout technique is based on the charge amplifier switched capacitor circuit. It is implemented using a commercially available microcontroller (EM microelectronics EM6819) which has the advantage of being available as single chips to enable post-processing steps such as backthining and chip embedding in a thin polymer package. Sensor and reference capacitors are homogeneously integrated on-foil. 400 and 30 µm thick microcontroller dies (MCU) are used in this application. The charge amplifier result is digitized using an internal 10-bit analog-to-digital converter (ADC). The 10-bit ADC is time multiplexed between the charge amplifier structure and the internal temperature sensor. Linear interpolation is used to fit the digital output of the ADC and calibrate the output of the sensor system. Readings of the humidity level and the temperature are written to an NFC tag (from the company EM microelectronics based on chip EM NF4) using the contact interface. Readings can be accessed using a customized android application on a smartphone.