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Compositional dependence of spontaneous emission probabilities between initial 4F,, and terminal ' Z, (J = 9/2, 1112, 13/2,15/2) levels of Nd3+ were studied for about 90 samples of silicate, borate, and phosphate glasses using the Judd-Ofelt theory. The effect of the covalency of the Nd-0 bond on the magnitude of intensity parameters was estimated from the variation of spectral profiles of the ' Z9/, 4 4G, , 'G,,, and 4F,12, 4S,,2 transitions. Intensity parameters R4 and R, and the spontaneous emission probabilities were strongly affected by the ionic packing ratio of the glass host. The results were discussed in terms of the site selectivity of Nd3+ ions in glasses.