The dielectric permittivity (ε) and the dc electrical resistivity(ρ)of pure LiNH4SO4 (LAS) single crystals have been measured in the temperature range 230-350 K including the lower phase transition at about 283 K. Anomalous behaviours before and at the transition point were observed. The temperature dependence of ρ yielded AE = 0.11 and 0.91 eV for the energy activating the charge transport mechanisms before and after the transition. A pre-transition phenomena was observed while measuring both ρ and ε. The optical transmittance at different temperatures was also measured and hence the absorption coefficient (α) and optical band gap Eopt,g were deduced. The validity of Urbach's rule was checked and hence the steepness parameter σ and the exciton energy E0 were calculated at different temperatures. ρ, ε, α and Eopt,g for LAS crystals doped with 5% Co2+ or Cu2+ were also measured and they were compared with the results of the pure crystals