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 delta N-15 and delta C-13 values of two freshwater fish species, Perca fluviatilis (perch) and Blicca bjoerkna (silver bream). Six-month preservation caused little enrichment in delta N-15 of both species compared to three month but had almost the same effects on delta C-13 values of both species as in three-month preservation. All methods caused significant shifts (enrichment) in delta N-15 of both species, and the effects in general were greater in perch (range: 0.28 parts per thousand-2.19 parts per thousand) than in bream (range: 0.31 parts per thousand-1.29 parts per thousand), which suggested that preservative induced shifts in delta N-15 was species-specific. The methods caused little enrichment (ethanol-range: 0.03 parts per thousand - 0.26 parts per thousand bream and 0.30 parts per thousand-0.48 parts per thousand perch and salt: 0.18 parts per thousand perch three month) and depletion (salt-range: 0.03 parts per thousand-0.13 parts per thousand bream and 0.13 parts per thousand perch six month) in delta C-13. Of the preservatives, however formalin had significant but consistent effects on delta C-13 in both species (-1.27 parts per thousand and -1.25 parts per thousand) for the entire preservation duration. Preservation-induced shifts in delta C-13 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 delta C-13 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 parts per thousand is required in ecological applications.