We show that death by dissolution is an important size-dependent mortality factor for juvenile bivalves. Utilizing a new experimental design, we were able to replicate saturation states in sediments after values frequently encountered by Mercenaria mercenaria in coastal deposits (Ω_aragonite = 0.4 and 0.6). When 0.2-mm M. mercenariawere reared in sediments at Ω_aragonite = 0.4 and 0.6, significant daily losses of living individuals occurred (14.0% and 14.4% d⁻¹, respectively), relative to supersaturated-control sediments (3.9% d⁻¹). For 0.4- mm M. mercenaria, significant mortality occurred under the most undersaturated conditions (Ω_aragonite = 0.4, mortality = 9.6% d⁻¹), although mortality at Ω_aragonite = 0.6 was not significant (mortality= 2.7% d⁻¹; control-saturated mortality = 0.2% d⁻¹). For the largest size-class investigated, 0.6 mm, we show significant mortality for clams under the most undersaturated sediments (Ω_aragonite = 0.4, 2.8% d⁻¹). To test if buffered sediments would increase survivorship of juvenile bivalves during periods of recruitment, we manually manipulated sediment saturation state by adding crushed Mya arenaria shell to a mud flat in West Bath, Maine, U.S.A. Although we increased the average sediment saturation state within retrieved cores from Ω = 0.25 ± 0.01 to only 0.53 ± 0.06, numbers of live M. arenaria in buffered sediment increased almost three-fold in 2 weeks. Buffering muds against the metabolic acids that cause lowered saturation states may represent a potentially important management strategy to decrease dissolution mortality.