The absolute polarity of single crystals of α‐Cu(IO3)2and Nd(IO3)3.H2O, determined by anomalous X‐ray scattering techniques, has been related to the sense of the piezoelectricd22and pyroelectricp2coefficients in these monoclinic crystals. The absolute sense ofd22in both cases is predictable from a consideration of the behavior of the spontaneous polarizationPsunder stress.Pswas calculated on the basis of a simple point‐charge and dipole model. The positive sense ofp2in α‐Cu(IO3)2and Nd(IO3)3.H2O (each with magnitudes of about 2 x 10−5Cm−2deg−1: the secondary pyroelectric coefficients are probably nearly an order of magnitude smaller) allows the inference that |Ps| increases with increasing temperature. The absolute sense ofd33in hexagonal LiIO3, determined by Morosin [Private communication (1972) quoted as Ref. 25 of Bergman, J. G.&Crane, G. R. (1974).J. Chem. Phys.60, 2470–2474] is similarly predictable from the calculatedPs. Turner's [J. Appl. Cryst. (1976).9, 52] measurement ofp3in LiIO3combined withPsalso leads to the inference of increasing |Ps|