The activation barriers for internal rotation around the C-N bond in p-substituted nitrosobenzenes were calculated using the density functional theory (DFT) and second-order Moller-Plesset (MP2) methods with the 6-31+g(d) basis set. The polarisable continuum model (PCM) was used to model the solvent effect. An explicit water molecule was also introduced to form a hydrogen bond with the nitroso group and its effect on the barrier was studied by DFT. The barriers were well-correlated with Hammett sigma(+) rather than sigma values, meaning a strong resonance effect on the transition state. The MP2 method produces better and comparable results with the few available experimental values.