Ab initio methods have been used to study the CH ... O hydrogen bond between H2O and CH3F, CH2F2, and CHF3. The calculations have been carried out using the 6-31G**, 6-311G**, 6-311++G**, and 6-31G** basis sets at the MP2 and MP4 levels of theory. Interaction energies include basis set superposition error adjustments. The results show that the strongest CH ... O hydrogen bond is the bisector one with the lone pairs on the oxygen. The inclusion of each additional fluorine results in a systematic strengthening of the hydrogen bond by 1 kcal/mol as well as in the shortening of the hydrogen bond distance by 0.1 Angstrom.