Numerous studies have reported on the positive effects of silicon (Si) on bone metabolism, particularly on the stimulatory effects of Si on osteoblast cells and on bone formation. Inhibitory effects of Si on osteoclast formation and bone resorption have also been demonstrated in vitro and are suggested to be mediated indirectly via stromal and osteoblast cells. Direct effects of Si on osteoclasts have been less studied and mostly using soluble Si, but no characterisation of the Si treatment solutions are provided. The aims of the present study were to (a) further investigate the direct inhibitory effects of Si on osteoclastogenesis in RANKL-stimulated RAW264.7 cells, (b) determine at what stage during osteoclastogenesis Si acts upon, and (c) determine if these effects can be attributed to the biologically relevant soluble orthosilicic acid specie. Our results demonstrate that silicon, at 50 μg/ml (or 1.8 mM), does not affect cell viability but directly inhibits the formation of TRAP+ multinucleated cells and the expression of osteoclast phenotypic genes in RAW264.7 cells. The inhibitory effect of Si was clearly associated with the early stages (first 24 hr) of osteoclastogenesis. Moreover, these effects can be attributed to the soluble orthosilicic acid specie.