Fluorescent lamps introduce a periodic interference signal in optical wireless receivers, which has the potential to degrade link performance severely. Usually, electrical high-pass filtering is employed to mitigate the effects of this interference, but this results in baseline wander or intersymbol interference. Digital pulse-interval modulation (DPIM) is a modulation technique which has been shown to be suitable for deployment in optical-wireless-communication systems. The effect of baseline wander on systems employing DPIM is examined. A new expression is given for the slot autocorrelation function of DPIM and, from this, the power spectral density is calculated. The error performance of DPIM, as a function of high-pass filter (HPF) cut-on frequency, is compared with the more established techniques of on-off keying using nonreturn-to-zero signalling (OOK) and pulse-position modulation (PPM), using both numerical analysis and computer simulation.