The effect of electric field on the diamagnetic and paramagetic muon spin rotation signals will be investigated for the organic molecular semiconductors NPB and TPBi . These materials find application as active layers in electronic devices such as the organic light emitting diode (OLED). The proposed measurements will complement a programme of low energy muon studies of active OLED devices recently begun at PSI in Switzerland. The key question to be addressed is whether the magnitude of internal electric field present near the heterojunction within the OLED is sufficently large to modify the balance of diamagnetic and paramagnetic states within the organic semiconductors. If so then implanted muons can be shown to provide a valuable new method of mapping out the internal electric field profile within an active organic electronic device.