Stable isotope is a powerful method for characterizing flows of energy through ecosystems. The power of this method, however, may be affected by preservation methods of the samples. We investigated the effects of four common preservatives (salt, formalin, and ethanol and freezing [control] and preservation duration (six and three months) on δ 15N and δ 13C values of two freshwater fish species, Perca fluviatilis (perch) and Blicca bjoerkna (silver bream). Six-month preservation caused little enrichment in δ 15N of both species compared to three month but had almost the same effects on δ 13C values of both species as in three-month preservation. All methods caused significant shifts (enrichment) in δ 15N of both species, and the effects in general were greater in perch (range: 0.28‰-2.19 ‰) than in bream (range: 0.31‰-1.29‰), which suggested that preservative induced shifts in δ 15N was species-specific. The methods caused little enrichment (ethanol-range: 0.03‰-0.26 ‰ bream and 0.30‰-0.48 ‰ perch and salt: 0.18 ‰ perch three month) and depletion (salt-range: 0.03 ‰0.13‰ bream and 0.13‰ perch six month) in δ 13C. Of the preservatives, however formalin had significant but consistent effects on δ 13C in both species (-1.27‰ and -1.25‰) for the entire preservation duration. Preservation-induced shifts in δ 13C were consistent in direction and magnitude for both species. The results suggested that ethanol and salt could be used without correction factor and formalin with correction factor for preservation of samples solely in δ 13C studies.. For the studies requiring use of carbon and nitrogen together, however, ethanol at least six month in preservation may be suitable for storing samples when considering detection of changes less than 2 ‰ is required in ecological applications. © by PSP.