Adding increasing quantities of an n-type compensating dopant, cobaltocene, to poly(3-hexylthiophene) reveals an almost perfect mirror symmetry between the conductivity and the luminescence intensity. The sharp minimum/maximum shows that the uncompensated p-type doping density is 1.2 x 10(18) cm(-3) and that excitons are strongly quenched by free charge carriers, not by bound charges.