Bismuth tetratitanate, Bi2Ti4O11, undergoes a paraelectric-to-antiferroelectric phase transition at about 233 degrees C. The integrated intensities of selected x-ray reflections and the linear optical excess birefringence have been measured as a function of temperature for Bi2Ti4O11 to characterize the transformation. Both methods indicate a continuous transition with a critical exponent of 1/4 . From the change of the lattice constants with temperature the scalar spontaneous strain, epsilon s, can be calculated. In the present case of a zone boundary transformation, epsilon s2 couples quadratically with the primary order parameter. The observed linear dependence of epsilon s2 on the temperature T points once again to the tricritical nature of the transition. Despite the mean field behaviour of the transition inferred from these results, diffuse intensities at the positions of superlattice reflections just above Tc may be attributed to the existence of non-classical order parameter fluctuations. Taking this observation into account the transformation can be designated as almost tricritical.