Electroluminescence in organic light-emitting diodes arises from a charge-transfer reaction between the injected positive and negative charges by which they combine to form singlet excitons that subsequently decay radiatively. The quantum yield of this process (the number of photons generated per electron or hole injected) is often thought(1) to have a statistical upper limit of 25 per cent. This is based on the assumption that the formation cross-section of singlet excitons, sigma (s), is approximately the same as that of any one of the three equivalent non-radiative triplet exciton states, sigma (T); that is, sigma (S)/sigma (T) approximate to 1. However, recent experimental(2) and theoretical(3) work suggests that sigma (S)/sigma (T) may be greater than 1. Here we report direct measurements of sigma (S)/sigma (T) for a large number of pi -conjugated polymers and oligomers. We have found that there exists a strong systematic, but not monotonic, dependence of sigma (S)/sigma (T) on the optical gap of the organic materials. We present a detailed physical picture of the charge-transfer reaction for correlated pi -electrons, and quantify this process using exact valence bond calculations. The calculated sigma (S)/sigma (T) reproduces the experimentally observed trend. The calculations also show that the strong dependence of sigma (S)/sigma (T) on the optical gap is a signature of the discrete excitonic energy spectrum, in which higher energy excitonic levels participate in the charge recombination process.