Trivalent oxides of yttrium and rare earth metals were evaluated as inert markers in apparent digestibility studies with salmonids in four experiments. In Experiment 1, 100 mg kg −1 of each of 15 oxides (Dy 2O 3, Er 2O 3, Eu 2O 3, Gd 2O 3, Ho 2O 3, La 2O 3, Lu 2O 3, Nd 2O 3, Pr 2O 3, Sc 2O 3, Sm 2O 3, Tb(III and IV) oxide, Tm 2O 3, Y 2O 3 and Yb 2O 3) were included in a feed fed to rainbow trout. The ratio between each marker and Yb 2O 3 in stripped faeces was used as an indicator of recovery. Only Er 2O 3, Ho 2O 3 and Tm 2O 3 had lower recoveries than the other markers. Experiment 2 compared the excretion rates of Cr 2O 3 and of the selected alternative markers (La 2O 3, Y 2O 3 and Yb 2O 3). A feed with 7.5 g kg −1 of Cr 2O 3 and 750 mg kg −1 of each of the other markers was fed to Atlantic salmon for a period of 1 week. Thereafter, the fish were fed with a marker-free feed, and gastro-intestinal evacuation was evaluated by comparing the marker ratios in the feed and in the faeces sieved from the outlet water of the tanks. The results did not reveal any systematic differences in evacuation among the various markers. Experiment 3 compared in vitro solubility of Cr 2O 3, Dy 2O 3, La 2O 3, Y 2O 3 and Yb 2O 3 in weak acid (HCl, pH 3 as in stomach contents of Atlantic salmon), weak acid neutralised with NaOH, and in water. Cr 2O 3 was not dissolved. Only 1.3% of Yb 2O 3, 22% of Y 2O 3, 31% of Dy 2O 3, and 96% of La 2O 3 was soluble in weak acid, but more than 99% of the dissolved markers precipitated when neutralised, and none of the markers were soluble in water. Experiment 4 compared the estimates of apparent digestibility coefficients (ADCs) of nitrogen and fat in rainbow trout when using Cr 2O 3, La 2O 3, Y 2O 3 and Yb 2O 3 as markers. The feed contained 10 g kg −1 Cr 2O 3 and 100 mg kg −1 of each of the other markers. Markers in feeds and stripped faeces were dissolved for the analysis with both HCl:HNO 3 and H 3PO 4:MnSO 4. Except for Cr 2O 3, the markers gave similar ADCs within each acid solubilisation procedure. The ADCs of fat were similar with both procedures, but the ADCs of nitrogen were 0.2% lower with HCl:HNO 3 than with H 3PO 4. Cr 2O 3 was incompletely dissolved in HCl:HNO 3, resulting in low ADCs. With H 3PO 4, no differences were seen among the ADCs obtained with Cr 2O 3 and the other markers. In conclusion, trivalent metal oxides, such as La 2O 3, Y 2O 3 and Yb 2O 3, can substitute Cr 2O 3 in digestibility studies with salmonids, and can be used at lower concentrations without affecting accuracy.