The final repository for short-lived, low and intermediate level radioactive waste in Sweden is built to act as a passive repository. Already within a few years after closure water will penetrate the repository and conditions of high alkalinity (pH 10.5-13.5) and low temperature (< 7 degrees C) will prevail. The mobility of radionuclides in the repository is dependent on the radionuclides distribution between solid and liquid phases. In the present work the incorporation of strontium (II) and europium (III) in alpha-calcium isosaccharinate (ISA) under alkaline conditions (pH similar to 10) at 5 degrees C and 50 degrees C have been studied. The results show that strontium and europium are incorporated into alpha-Ca(ISA)(2) when crystallized both at 5 degrees C and 50 degrees C. Europium is incorporated to a greater extent than strontium. The highest incorporation of europium and strontium at 5 degrees C rendered the phase compositions Ca0.986Eu0.014(ISA)(2) (2.4% of Eu(ISA)(3) by mass) and Ca0.98Sr0.02(ISA)(2) (2.2% of Sr(ISA)(2) by mass). XPS spectra show that both trivalent and divalent Eu coexist in the Eu incorporated samples. Strontium ions were found to retard the elongated growth of the Ca(ISA)(2) crystals. The incorporation of Sr2+ and Eu3+ into the solid phase of Ca(ISA)(2) is expected to contribute to a decreased mobility of these ions in the repository.