The sun protection factor (SPF) refers to the level of protection of a sunscreen against UV rays and is expressed, according to the COLIPA recommendation as a numerical value, and in terms of low, medium, high, and very high protection as well. For the determination of SPF exist methods in vivo (on volunteers) and in vitro (laboratory tests on artificial substrates). At European level there is a standard method for the determination of “in vivo” SPF: "International Sun Protection Factor Test Method " and a standard method to determine the “in vitro” UVA-PF: “Method for “in vitro” Determination of UVA protection " published by COLIPA in 2006 and 2011 respectively. There is still no definitive and optimized procedure to analyze the “in vitro” SPF. In this work, it was assessed the influence of substrate type and amount of product applied on the determination of “in vitro” SPF. The measurements were carried out with two instruments: Labsphere UV 1000S and UV 2000S, in order to assess their equivalence for such determinations. The study also focused on the evaluation of UVA-PF on several commercially available substrates. With the two instruments it is possible to get results “in vitro” correlate with those “in vivo” for low and medium SPF, while for high values of SPF some problems may occur. We compared SPF “in vivo” values with those measured “in vitro”. From all analyses we were able to point out that, as regards SPF determination the Transpore is an excellent substrate for the “in vitro” determination excluding alcoholic or high lipophilic preparations. Values even more similar to those “in vivo” were obtained with two different types of PMMA having same roughness (Ra). With regard to the UVA-PF two different roughness of PMMA were tested .The best results were obtained with the PMMA of Ra: 4.8. Of all the emulsions tested it was obtained a value of UVA-PF exceeding 1/3 of SPF calculated in vivo.