Hole transport molecules have long been exploited for xerographic photoreceptors and recently in OLED's, photovoltaic cells, or photorefractive polymers. Hole mobilities in substituted N, N'-bis-(m-tolyl)-N-N'-diphenyl-1,1'-biphenyl-4,4'-diamine (TPD) derivatives doped in polystyrene (PS), were analyzed by the time-of-flight technique to determine the effect of altering the geometric and electronic structure of TPD. Data were collected as a function of applied field and temperature to yield the energetic and positional disorder parameters defined in the disorder formalism. The impact of the molecular dipole moment on transport properties was also evaluated. The larger molecular dipole moments of the derivatives correspond to an increase in the energetic disorder, which contributes to their lower mobilities.