The aim of the present study was to investigate the photochemical behavior of DHHB and its photostabilizing effect on avobenzone (AVO) in different sunscreen formulations. The formulations were subjected to photostability studies by HPLC and spectrophotometry. In vitro phototoxicity was assessed using 3T3 fibroblast cultures. The mechanism of interaction between DHHB and AVO was investigated by steady state and time-resolved fluorescence spectroscopy. All formulations provided ultra-protection against UVA radiation. HPLC results demonstrated that DHHB did not present a photostabilizing effect on AVO. Fluorescence spectroscopy showed that AVO and DHHB interact by a static quenching mechanism and DHHB did not affect the AVO excited state lifetime. In addition, the energy transfer by Förster mechanism (FRET), which is the most often mechanism responsible for singlet-singlet quenching, is unlikely in this work. These results suggest why DHHB did not work as a photostabilizer on AVO singlet excited state. Phototoxicity results demonstrated that combinations containing DHHB (C2) did not show a phototoxic potential. Finally, although DHHB was considered to be photostable for all formulations studied (F2 and F3) it did not increase the photostability of AVO (F3). Thus, we suggested that formulations containing DHHB (F2) should be considered more advantageous than formulations containing AVO and AVO/DHHB (F1 and F3 respectively).