New liquid crystalline substance, (S) 4-(1-methylheptyloxycarbonyl)-3-chlorophenyl 4'-(4-butanoiloxybut-1-oxy)biphenyl-4-carboxylate, exhibiting ferroelectric and antiferroelectric phases have been studied by means of frequency domain dielectric spectroscopy. DSC calorimetric and electrooptic measurements were done to study its phase transitions and electrooptic behaviour. In the antiferroelectric SMCA* phase one of the dielectric relaxation modes exhibits a non-Arrhenius behaviour. The plots of critical frequencies and dielectric increments vs. temperature show an anomalous behaviour of the relaxation processes in the neighbourhood of the ferroelectric - antiferroelectric phase transition. Reversal current method reveals one current peak in the ferroelectric phase and two in the antiferroelectric one, Temperature dependencies of tilt angle, spontaneous polarization, switching time and transmittance are also presented and discussed in scope of the mean-field theory.