Widespread utilization of silicon isotope ratio variations (d29Si and d30Si) as a geochemical and paleoceanographic tracer has been hampered by the hazardous nature of the analysis, which requires the use of a fluorinating gas. Multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) provides a safer means of silicon isotope analysis. High backgrounds, long rinsing and sample introduction times, and interferences are significant difficulties. It is possible to work around these obstacles, and the measurement of d29Si may be done to a precision averaging ± 0.1%. The d30Si, which cannot be measured directly due to a NO interference at mass 30, may be extrapolated from the theoretically derived and empirically verified relationship, d30Si = 1.93d29Si. Biological materials (sponge spicules and diatoms) measured via MC-ICP-MS yielded d30Si values in the ranges expected from the prior analysis of similar samples by the method of fluorination.