The field-effect method in electrolyte was used to study the zero-gap semiconductor HgSe-electrolyte (saturated solution of KCl) system by measuring the capacitance-voltage and current-voltage characteristics. A technique for the estimation of the matrix element P of the quasi-momentum operator from capacitance-voltage characteristics was proposed, and the value of P for HgSe was determined at T = 295 K. © 2002 MAIK "Nauka/Interperiodica".