Introduction A MC-ICP-MS (Multi Collector Inductively Plasma Mass Spectrometer from Nu Instruments) has been tested to precisely detect variations in the isotopic composition of Si on several samples. Results and Discussion Variation in sample 28Si/29Si ratios are expressed as δ29Si units, which represent deviations in ‰ from the same ratio in a sample relative to a quartz standard measured using the bracketing “standard-sample-standard” technique. Taking into account the interferences of N2, CO and NO in the Si mass region, and measuring in dynamic mode the Mg isotopes as internal standard, the repeatability on the δ29Si measurements is better than 0.15 ‰ and its individual precision better than 0.05 ‰. Its accuracy is estimated by inter-laboratory comparison. Factors that are found to strongly affect the precision and accuracy of isotope ratio measurements by MCICP-MS are mass discrimination of interferences, drift of the isotope ratio and matrix effects. We will present the results obtained on natural silica and opal (quartz, diatoms, sponges) and artificial samples. We will discuss the various analytical methodologies tested (e.g., dry/wet plasma, sample dissolution, Si/Mg ratios, sensitivity). Conclusion These results are of great relevance regarding environmental and geological sciences where Si isotopes could open up a range of new analytical studies. Moreover, the MC-ICP-MS technique has obvious advantages over previous mass spectrometry techniques used to determine Si isotope ratios. The sample introduction system does not request handling dangerous chemicals and the instrument presents very good precision and sensitivity.