New features in electron paramagnetic resonance (EPR) of porous silicon have been examined here. A new isotropic EPR center was observed atg=1.9995(1) atT=4.2 K, in bothp-type andn-type porous silicon. By comparing itsgvalue with those of shallow donors in bulk silicon, the center was identified due to the conduction-band (CB) electrons in silicon microcrystals. The CB signal, present in freshly preparedp-type andn-type samples, can be dramatically and surprisingly enhanced by the presence of a polar solvent on then-type porous silicon surface. Even though it was shown that most of the donor electrons in ann-type sample can be pulled into the porous layer from the substrate by solvent exposure of the porous layer, the possible electrochemical effects are not yet completely understood; to establish a reasonable model for them would require appropriately controlled experiments. ©1997 American Institute of Physics.