A thin-layer synthesis technique was used to synthesize bulk amounts of the metastable phase, RuSb3, a novel compound with the skutterudite structure. The compound crystallized at 350 degreesC and was stable to 525 degreesC. When annealed above 550 degreesC, it decomposed into RuSb2 and Sb. Rietveld refinement of X-ray diffraction data showed the presence of excess Sb residing in the interstitial site in the skutterudite structure. X-ray diffraction and thermal analysis experiments allowed us to examine the evolution of the sample as a function of annealing and determine the reaction pathway. The activation energy for the crystallization of the compound was determined to be 3 eV/ nucleation event, while the activation energy for decomposition was approximately 8 eV.