The high instability and hysterisis of the relative humidity sensors currently available on the market render necessary simple and economic calibration methodologies that can be used as a secondary of working standards. The chemical equilibrium-type systems based on saturated aqueous salt solutions, even though simple and economical, are not always metrologically satisfactory for calibration. They can, in fact, be unreliable, when some fundamental requirements are neglected: also, unacceptable discrepancies continue to exist in the equilibrium relative humidity reference data of saturated aqueous salt solutions furnished by both literature and standards. To highlight the factors that increase the reliability of calibrations with saturated aqueous salt solutions, the authors of this paper have redetermined the equilibrium relative humidity reference data of 11 saturated aqueous salt solutions at ambient pressure and temperature. The solutions chosen were the ones generally used as fixed points to obtain a relative humidity calibration scale.