The isotopic invariance criterion, ab initio molecular orbital calculations and principal component analysis are used to determine polar tensor values from the experimental infrared intensities of methyl chloride, bromide and iodide. For the A1 symmetry species the preferred dipole moment derivative sign combinations (+ - -)H (+ - -)D for all three halides are in agreement with previous studies. On the other hand the E symmetry species (+ - +)H (+ - +)D combinations determined in this work are not the same as those reported previously. The carbon mean dipole moment derivative values vary linearly with the average electronegativities of the terminal atoms, whereas the hydrogen and halogen mean derivatives are linearly dependent on their own electronegativity values.971743544359