The isotopic effect on the magnitude of quadratic electrooptic coefficients
in KDP is investigated. By employing both the Miller rule and assumption o
f a rough constancy of intrinsic electrooptic coefficients, one can expect
these quadratic electrooptic coefficients which are related to the electric
field applied in the [100] direction, i.e. the coefficients g(1111), g(221
1) and g(3311) to be in DKDP almost twice larger than that for KDP. Our mea
surements of the relative quadratic coefficient n(0)(3)g(1111)-n(e)(3)g(331
1) in DKDP are in contradiction with the prediction. We found g(1111), g(22
11) and g(3311) to be weakly dependent on the deuteration.