The gas phase (1 atm, 453 K) hydrogenation of p-chloronitrobenzene (p-CNB) over a series of laboratory synthesised and commercial (activated carbon (AC), non-reducible (SiO2 and Al2O3) and reducible (ZnO) oxide) supported Pd (1-10 % wt.) has been examined. Laboratory synthesis involved standard impregnation and deposition of monodispersed nano-scale Pd0 particles. Reaction over these catalysts generated the target p-chloroaniline (p-CAN) (via selective hydrogenation) and nitrobenzene (NB)/aniline (AN) as a result of a combined hydrodechlorination/hydrogenation. A range of mean Pd particle sizes (2.4-12.6 nm) exhibiting different electronic character (from XPS analysis) was generated. Particle size was determined by electron microscopy and (H2 and CO) chemisorption. p-CNB consumption rate and H2 chemisorption both increased with decreasing Pd particle size. The presence of residual Mo from the Na2MoO4·H2O stabilizer in catalyst synthesis by colloidal deposition suppressed activity but this was circumvented by the use of poly N-vinyl-2-pyrrolidone (PVP) as stabilizer. Pd/AC generated p-CAN and AN as principal products, Pd on SiO2 and Al2O3 exhibited hydrodechlorination character (generating AN and NB) while Pd/ZnO promoted the sole formation of p-CAN at all levels of conversion. Reaction selectivity is linked to Pd site electron density where the formation of Pdδ+ (on AC) activates the nitro group with subsequent C-Cl bond scission and the occurrence of Pdδ- (on SiO2 and Al2O3) favours interaction via the aromatic ring that activates both –NO2 and –Cl for attack. Reaction exclusivity to p-CAN over Pd/ZnO is attributed to the formation of supported PdZn (demonstrated by XPS), which selectively activates the –NO2 group. This is the first report that demonstrates 100% selectivity for p-CNB → p-CAN over supported Pd.