The spectral phase‐matching bandwidths in LiIO3, LiNbO3, CDA, and KDP (type I and type II) for second harmonic generation (SHG) at 1.06 &mgr;m have been accurately measured using a tunable line‐narrowed Nd : glass laser. The present results disagree with previously published data. Mechanisms such as sum‐frequency generation which could contribute to the discrepancy are discussed and demonstrated in a 90° phase‐matched deuterated CDA crystal.