Second‐harmonic generation (SHG) properties of two novel organic crystals, i. e., 2,5‐ bis(benzylidene)cyclopentanone (BBCP) and 2,6‐bis( p‐methylbenzylidene)‐4‐tert‐butylcyclo‐ hexanone (MBBCH), were studied. Based on an x‐ray structural analysis, their SHG coefficientsdijat 1.06 &mgr;m are determined by the Maker‐fringe method asd14=7 pm/V for BBCP, andd31=15 pm/V,d32=12 pm/V, andd33=4 pm/V for MBBCH, respectively. Measurements on the refractive indices reveal that angle‐tune phase matching in the form of both types I and II is possible, in particular, with a large efficiency for MBBCH,deff∼12 pm/V, which was confirmed experimentally. ©1995 American Institute of Physics.