Metabolic acidosis induces resorption of cultured bone, resulting in a net efflux of calcium (Ca) from the bone and an apparent loss of mineral potassium (K). However, in these organ cultures, there is diffusion of K between the medium and the crystal lattice, causing difficulty in interpretation of the acid-induced changes in mineral ion composition. To determine the effects of acidosis on bone mineral K, we injected 4-day-old neonatal mice with pure stable isotope41K, equal to ∼5% of their total body K. Calvariae were dissected 24 h later and then cultured for 24 h in medium without added41K, either at pH ∼7.4 (Ctl) or at pH ∼7.1 (Ac), with or without the osteoclastic inhibitor calcitonin (3 × 10−9 M, CT). The bone isotopic ion content was determined with a high-resolution scanning ion microprobe utilizing secondary ion mass spectrometry.41K is present in nature at 6.7% of total K. The injected41K raised the ratio of bone41K/(39K+41K) to 9.8 ± 0.5% on the surface (ratios of counts per second of detected secondary ions, mean ±95% confidence interval) but did not alter the ratio in the interior (6.9 ± 0.4%), indicating biological incorporation of the41K into the mineral surface. The ratios of41K/40Ca on the surface of Ctl calvariae was 14.4 ± 1.2, indicating that bone mineral surface is rich in K compared with Ca. Compared with Ctl, Ac caused a marked increase in the net Ca efflux from bone that was blocked by CT. Ac also induced a marked fall in the ratio of41K/40Ca on the surface of the calvariae (4.3 ± 0.5, p < 0.01 vs. Ctl), which was partially blocked by CT (8.2 ± 0.9, p < 0.01 vs. Ctl and vs. Ac), indicating that Ac causes a greater release of bone mineral K than Ca which is partially blocked by CT. Thus, bone mineral surface is rich in K relative to Ca, acidosis induces a greater release of surface mineral K than Ca, and osteoclastic function is necessary to support the enriched levels of surface mineral K in the presence of acidosis.