It is well-recognized that aldimines are less reactive than aldehydes toward nucleophilic additions. In this paper, an unprecedented change in the reactivity is described: preferential reactions of aldimines over aldehydes in nucleophilic additions using a lanthanide salt as a Lewis acid catalyst. In the presence of a catalytic amount of ytterbium triflate (Yb(OTf)(3)), only aldimines reacted with silyl enol ethers, ketene silyl acetals, allyltributylstannane, or cyanotrimethylsilane to afford the corresponding adducts in high yields, even in the coexistence of aldehydes. Selective formation of an aldimine-Yb(OTf)(3) complex rather than an aldehyde-Yb(OTf)(3) complex was indicated by C-13 NMR analyses. While this report demonstrates the effective use of Lewis acids in organic synthesis, the basic idea of changing reactivity as shown here will be widely applied to many other nucleophilic additions.