The estimation of biomass from body lengths to carbon regressions is a common approach in plankton research. Several different chemicals for sample preservation are in use, and conversion factors to account for shrinkage effects exist, but to our knowledge the consequences of using potassium-iodide and iodine (Lugol solution) as preservative on body sizes of different mesozooplankton groups have not been investigated. We tested the effect of 2% acidified Lugol solution on body sizes over time on two major marine mesozooplankton groups, namely larvaceans and copepods, which are representatives of gelatinous and chitineous plankton, respectively. For both, we observed a significant shrinkage effect over time (P <0.0001). Larvaceans showed a reduction in body size by 20% within the first 2 min, which stabilized after 36 h at 22%, whereas copepods shrank by 17%. These differences were significant (P = 0.0324), with no further shrinkage observed over an additional 3 months. Failure to adequately account for shrinkage effects could result in significant biomass underestimation if length-weight relationships are generated from live material.