Collisional rate coefficients for rotational transitions in C3H2 and SiC2 due to collisions with H2 molecules are calculated. The C3H2 has two distinct species, ortho and para, whereas the SiC2 has only ortho specie due to spin statistics for the identical carbon nuclei. For ortho- and para-C3H2 we accounted for 47 and 48 energy levels, respectively. The calculations are done for the kinetic temperatures 30, 60, 90, and 120 K. For ortho-SiC2, we accounted for 40 energy levels and the calculations are done for the kinetic temperatures 25, 50, 75, 100, and 125 K. The results are reported here for the transitions in the upward direction. The values for the downward direction can easily be calculated with the help of the detailed equilibrium equation.