Although atmospheric 222Radon (222Rn) activity concentration measurements are currently performed world-wide, they are being made by many different laboratories and with fundamentally different measurement principles, so compatibility issues can limit their utility for regional-to-global applications. Consequently, we conducted a European‐wide 222Rn/222Rn progeny comparison study in order to evaluate the different measurement systems in use, determine potential systematic biases between them, and estimate correction factors that could be applied to harmonize data for their use as a tracer in atmospheric applications. Two compact portable Heidelberg Radon Monitors (HRM) were moved around to run for at least one month at each of the nine European measurement stations included in this comparison. Linear regressions between parallel data sets were calculated, yielding correction factors rela tive to the HRM ranging from 0.68 to 1.45. A calibration bias between ANSTO (Australian Nuclear Science and Technology Organisation) two‐filter radon monitors and the HRM of ANSTO/HRM = 1.11±0.05 was found. Moreover, for the continental stations using one‐filter systems that derive atmospheric 222Rn activity concentrations from measured atmospheric progeny activity concentrations, preliminary 214Po/222Rn disequilibrium values were also estimated. Mean station-specific disequilibrium values between 0.8 at mountain sites (e.g. Schauinsland) and 0.9 at non‐mountain sites for sampling heights around 20 to 30 m above ground level were determined. The respective corrections for calibration biases and disequilibrium derived in this study need to be applied to obtain a compatible European atmospheric 222Rn data set for use in quantitative applications, such as regional model intercomparison and validation, or trace gas flux estimates with the Radon‐Tracer‐Method.