This paper presents a formulation of seminumerical contact transformations for rovibrational spectroscopy. Effective rotational Hamiltonians are obtained starting from a rovibrational Hamiltonian with an exact kinetic energy operator in curvilinear internal valence coordinates. Like the accuracy of the variational methods, the accuracy of this method can be increased by using more computational power. Error estimates are also calculated. Main motivations for using seminumerical contact transformations in rovibrational spectroscopy are considered. As an example, a calculation is carried out for H2S. No remarkable deviations between the calculated and the observed effective constants were observed for the states considered (ground states, nu(2) nu(1), nu(3), 2 nu(2), 2 nu(1) + nu(2), nu(1) + nu(2) + nu(3), nu(2) + 2 nu(3)) New ideas for further research of this subject are suggested.