The paper presents a theoretical analysis to assess the effect of temperature and cable length on TDR signal travel time and, hence, on water content measurement. In addition, the effect of multiple reflections occurring at the probe head is analysed. Probe heads of current commercially available probes are formed by two series impedances which cause the waveform to first descend and then ascend when the signal travels along the probe head. This form of the wave at the interface between cable and soil is somehow different from the classical ‘single rising limb’ form (single-impedance head) reported in the literature (Heimovaara & Bouten 1990). A question that might be asked is whether the methods for travel time determination developed for single-impedance heads still hold for double-impedance heads.