Comparative studies of the electrochemical behaviour of the growth and nucleation surfaces of a free-standing boron-doped polycrystalline diamond film grown in a microwave plasma CVD reactor are performed. The uncompensated acceptor concentration in diamond is determined from the electrochemical impedance (Mott-Schottky plots), uncompensated boron acceptor concentration from infrared absorption measurements, and the total boron concentration, by the SIMS method. In the diamond bulk adjacent to the nucleation surface, constituted from submicrometre-sized crystallites, both the boron concentration and the total acceptor concentration are found to be significantly higher than near the growth surface, where the film crystallinity is more perfect. This difference is tentatively attributed to the increased concentration of crystal lattice defects near the nucleation surface. These defects, in addition to boron atoms, play the role of acceptors in diamond.