A metastable (face centered) cubic form of indium has been identified in melt-spun Al-4.3 at% indium using high resolution synchrotron radiation diffraction. The microstructure of the melt-spun ribbons consisted of particles of nominally pure indium in an aluminum matrix. These particles ranged in size from 10 nm to 1000 nm through the thickness of the ribbons. Only the smallest indium particles were identified as being cubic and these were observed in electron transparent regions of the as-spun ribbons. The cubic indium phase had the following orientation relationship with the matrix: [001]In′ ∥ [001] Al and (100)In′ ∥ (001) Al. The size distribution and average diameter of the cubic particles were obtained by quantitative analysis of the diffraction line profiles. These measurements were corroborated with transmission electron microscopy observations. The volume percent of the cubic indium phase was determined by matching simulations of the x-ray diffraction patterns with the diffracted intensities found experimentally. Approximately 10-20% of the indium is present in the cubic form and the remainder is the stable tetragonal phase.