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We report on the nonlinear optical properties of a new class of organic compounds built of trihalides interacting weakly with donor type molecules. The adducts of iodoform with sulphur (CHI3.3Sa) and iodoform with quinoline (CH13.3C9H7N) exhibit sizeable electrooptic effects and second harmonic generation comparable to those of the most efficient organic coumpounds such as MNA (methylnitroaniline). For both iodoform compounds the angle-tuned phase matching of type I and I1 has been observed. The comparison between the electrooptic effect and the second harmonic generation suggests that in these materials the electrooptic effect is mostly electronic in origin. This permits the construction of high speed electrooptical modulators and switching devices operating at very high frequencies up to the terahertz region. The macroscopic optical nonlinearity of the adducts stems mainly from the nonlinearity of the iodoform molecule. The largest electrooptic coefficients (r22 = 2.2pm/V) and the largest nonlinear optical susceptibility (d22 = lS.lpm/V) were measured in the iodoform-sulphur compound.