A solidification boundary layer, where vorticity is concentrated, is shown to scale as δ ∼ (v/V) sin θ where θ is the angle that the solidification front makes with the oncoming flow, v is the kinematic viscosity of the melt, and V is the solidification rate. Key similarities with the suction boundary layer are outlined. Transport of vorticity, by diffusion and advection, in flows that solidify is discussed in the context of the planar-flow spin casting process, with special attention paid to how vorticity leaves the flow. Direct experimental evidence for a solidification layer in that application is reported.