Description: Stable isotope analysis (SIA) is used to investigate the trophic ecology of organisms. After field sampling, tissues are usually preserved dried or frozen. However, samples not initially collected for SIA such as museum samples are often fixed in preservative fluids. Nevertheless, preservative fluids may alter stable isotope ratios and, potentially, diverse metrics derived from isotopic data (i.e. isotopic niches). Consequently, their effects should be quantified to determine whether museum samples are suitable for SIA. This dataset is the result of the experimental assessment of the long-term effects (up to 24 months) of freezing, drying, formaldehyde and ethanol preservation on δ13C, δ15N, δ34S values, C/N ratios and on isotopic niche parameters of the sea star Marthasterias glacialis (Echinoderm). Formaldehyde quickly affected δ13C values. However, after being altered, the mean δ13C value remained stable over time, suggesting that a correction factor could be used regardless of preservation time. Ethanol induced a gradual increase of δ13C values until an asymptote that could also be adjusted with a correction factor. None of the preservation methods had significant or consistent effect on δ15N. Formaldehyde induced a decrease of δ34S values that could be adjusted by adding correction factors. The parameters of the isotopic niches were slightly modified across time when computed with δ13C and δ15N values, but inconsistent variations occurred when computed with δ13C and δ34S values. Overall, these results show that correction factors may be used to mitigate the effects of the preservation method on stable isotope ratios of sea stars and that, in most case, museum samples are suitable to calculate isotopic niche parameters.