The publication of the CEN/TS 772-22 in 2006 as European method for determining the frost resistance of clay masonry units has allowed each country to gain experience with the procedure and to evaluate its suitability in comparison with the existing local provisions. Therefore, in Belgium, a prenormative project has been launched by BBRI in collaboration with BCRC as support of both the Belgian and the European standardization commissions ‘Masonry products' (CEN TC 125). The first part of the study had shown the lack of severity of the European test method in comparison with the Belgian one and has led to a negative vote from Belgium to convert the TS into an EN in 2009, and thus to the preservation of the existing Belgian provisions. In order to develop and to harmonize one suitable test procedure without reducing the current quality level, the second part of the project has been focussing on optimizing the European method and reinforcing its severity by means of an experimental parametric study and quantitative methods. This paper presents the obtained results. Furthermore, monitored natural exposure sites were designed and built to obtain realistic data on the observed water content, temperature profile and strength losses of the bricks, as a function of the exposure levels.