This paper reports an experimental data analysis which clearly emphasizes the complex nature of the mechanisms governing the mixing of passive scalars such as temperature in fully developed turbulence. For that purpose, we compare our measurements of temperature increments and their probability density functions (pdfs) to theoretical predictions available in the literature for a scaler field evolving within a fully stationary developed dynamical turbulent field. The observed disagreements lead us to propose some improvements of the existing models. This, in fact, underlines the evolution through the scales of turbulence of the statistics of temperature increments, whose coupling with the velocity field appears to be an essential feature of the mixing process.