We report on an experimental study of spin-dependent exciton formation in films of π-conjugated polymers. We employed a spectroscopic/magnetic resonance technique, namely the photoinduced absorption (PA)-detected magnetic resonance technique, and showed that non-geminate recombination pairs of polarons form singlet excitons more efficiently than triplets. The experimental proof for this is achieved by detection of a resonant decrease of triplet exciton absorption and resonantly enhanced singlet photoluminescence (PL). In addition, resonantly enhanced absorption is observed which we assign to a singlet exciton precursor. Our study has important implications regarding the efficiency that can be achieved in polymer light-emitting diodes (PLEDs).