The properties of electron−hole (e−h) pairs generated in a working poly(3-hexylthiophene) (P3HT) diode are investigated by electrically detected magnetic resonance (EDMR) techniques. The EDMR intensity is shown to increase with increasing density of injected electrons for a given hole density, demonstrating that the EDMR signal arises from an e−h pair. The EDMR spectrum consists of two Gaussian curves, one of which gives a g-value very similar to that of hole carriers, suggesting that the EDMR spectrum is given by the sum of electron spin resonance (ESR) spectra from hole and electron carriers forming e−h pairs. Bias-dependent correlation between carriers and e−h pairs under diode operation is examined directly from independent measurements of near-infrared (NIR) spectroscopy for carriers and EDMR for e−h pairs. When the bias is increased to above a threshold, the EDMR signal is strongly reduced despite a gradual increase in the NIR signals, providing evidence that the e−h pairs are dissociated by an electric field in the bias region. Because of this property of the pair, the correlation between the carriers and e−h pairs inside the diode changes depending on the bias magnitude: the carriers and e−h pairs coexist or only the carriers exist. The e−h pair is presumed to be formed across different lamellae because of the crystalline phase of the P3HT film. Although the intensity of the EDMR signal from the interlamellar e−h pair is small, the intensity increases substantially when the sample is exposed to air, suggesting the formation of a magnetoreactive species that incorporates oxygen.