The first electrochemical investigation of composites materials based in polyoxometalates (POMs) encapsulated in the porous Metal-Organic Framework (MOE) MIL-101 is reported. The electrochemical behaviour of the porous MIL-101 and composite materials based in two monovacant Keggin-type POMs {K-7[PW11O39]center dot 10H(2)O (PW11) and K-8[SiW11O39]center dot 13H(2)O (SiW11)} encapsulated in MIL-101 (respectively, PW11@MIL-101 and SiW11@MIL-101) were studied by cyclic voltammetry. Initially, to investigate the influence of the support MIL-101 in the electrochemical behaviour of the two monovacant POMs, the compounds were individually immobilized on the surface of a pyrolytic graphite (PG) electrode. A reversible surface-controlled redox signal attributed to the chromium metal centres (Cr3+/Cr2+) was identified in MIL-101, and this represents the first electrochemical study of this MOP material. The monovacant compounds PW11 and SiW11 also revealed surface-controlled processes corresponding to W-VI <-> W-V reductions. PW11 showed two reversible signals (involving one or two electrons), whereas SiW11 showed a quasi-reversible one-electron and a reversible two-electron process for the first and second W reductions, respectively. For the latter, an extra process was detected and attributed to the formation of beta-isomer (beta-SiW11). When the composites materials PW11@MIL-101 and SiW11@MIL-101 were immobilized on the surface of the electrode, the support MIL-101 maintained its characteristic electrochemistry and the redox potentials of the POMs were not significantly changed. However, when encapsulated inside the cages of the support material the POMs revealed a distinct electrochemical behaviour, given that their redox processes became diffusion-controlled, indicating that the POMs have some degree of mobility inside the support framework. (C) 2012 Elsevier Ltd. All rights reserved.