J. Cosmet. Sci., 63, 243–254 ( July/August 2012) 243 A new chemical approach to optimize the in vitro SPF method on the HD6 PMMA plate S. MARGUERIE, M. PISSAVINI, A. BAUD, T. CARAYOL, and O. DOUCET, Lancaster–Coty, International Research & Development Center, Athos Palace, 2 Rue de la Lujernetta, MC 98000 Monaco. Accepted for publication December 21, 2011. Synopsis In a previous study, we demonstrated that control of the roughness of molded PMMA plates improves in vitro SPF reproducibility. However, in vitro/vivo deviations are still observed. Sunscreens show different behavior during spreading on the HD6 surface according to the formulation, re- sulting in a more or less homogenous distribution. The hydrophilic nature of HD6 appears to contribute signifi cantly during spreading. Two different sunscreens offering a homogenous and non-homogenous distri- bution were investigated to check if the interfacial tension between product and substrate has a real infl uence on the spreading quality. Using microscopic observations, we attempted to correlate the in vitro SPF results with the product’s spreading property. In order to reduce this interfacial tension, an HD6 pretreatment with an amphoteric surfactant, cocamido- propyl betain, was performed. In vitro SPF on “pretreated HD6” was examined using a cohort of 30 products. This pretreatment led to reliable results, demonstrating good association with the in vivo SPF. INTRODUCTION The challenge facing the cosmetics industry is to develop innovative, effi cient, sunscreen products that conform with European recommendations with a minimum of time and cost. In this regard, a reliable spectroscopic in vitro method is an essential tool. The advantages are well known. It is fast and relatively inexpensive, and moreover it offers complete spec- tral information on products. And importantly, there are no ethics problems (1–3). The EU recommendation on sun protection products, published in September 2006, recommends the use of in vitro methods. Although Colipa succeeded in establishing a UVA in vitro method (1), the in vitro SPF determination remains a challenge. Indeed, different working groups such as ISO, DGK, and even Colipa concentrated their efforts on devel- oping an in vitro reliable method with little success. All the ring tests performed led to the same conclusions: whatever the method used or the parameters chosen, inter-laboratory variability remains a major problem. The actual situation is the following: The in vitro SPF determination is accessible to every laborator, but the lack of control of the variables infl uencing the results can lead to poor results. The technique is very sensitive to different parameters (4). This may explain the