The gas phase hydrodechlorination (HDC) of chlorobenzene (CB), 1,2-dichlorobenzene (1,2-DCB) and 1,3-dichlorobenzene (1,3-DCB) has been investigated over Pd/SiO2 and a series of Ln-Pd/SiO2 prepared from the organometallic precursor{(DMF)(10)Ln(2)[Pd(CN)(4)](3)}infinity where Ln =La, Ce, Sm, Eu, Gd and Yb; Pd loading = 5%, w/w. Under identical reaction conditions, the following overall sequence of increasing initial fractional dechlorination has been established: Pd/SiO2 < Yb-Pd/SiO2 approximate to Sm-Pd/SiO2 < Gd-Pd/SiO2 < La-Pd/SiO2 approximate to Ce-Pd/SiO2 < Eu-Pd/SiO2; reaction over Ln/SiO2 resulted in a negligible conversion. HDC activity declined with time-on-stream but the Ln-Pd/SiO2, catalysts maintained a significantly higher fractional HDC than Pd/SiO2; loss of activity is attributed to deleterious HCl/surface interactions. The pre- and post- reaction catalyst samples have been characterized in terms of BET area, TPR, TEM, H-2 chemisorption/TPD, XRD and XPS analyses. When compared with Pd/SiO2, Pd is present in the Ln-Pd/SiO2 samples as much smaller particles, while the lanthanide component is finely dispersed over the surface, i.e. Ln is in intimate contact with Pd. The promotional effect of Ln in Ln-Pd/SiO2 is attributed to a surface Pd/Ln synergism resulting in an enhancement of surface reactive hydrogen and a more effective C-Cl bond activation for hydrogenolytic attack. HDC performance is discussed in terms of surface composition, Pd particle size, Ln electronic structure and H2 uptake/release dynamics. (c) 2007 Elsevier B.V. All rights reserved.