The optical transmittance for potassium ferrocyanide (KFCT) single crystals were measured as a function of the wavelength at different temperatures in the range 275–320 K along the two axes (I 0 ) and (0 1 0) and hence the absorption coefficient (α) and optical band gap Eg were deduced. The type of transition was determined and the validity of Urbach's rule was checked. The steepness parameter (σ) and the exciton energy E0 were calculated at different temperatures. The temperature dependence of the energy gap was estimated and the result was confirmed through the calculation of dEg/dT from information about σ. The refractive index, the extinction coefficient and both the real and imaginary parts of the dielectric permittivity were also calculated as a function of photon energy and the results were discussed. The effect of doping KFCT with 2% of sodium nitroproced, cobalt sulphate or potassium dichromate on the same physical parameters was also discussed