The dipole moments of a number of substituted benzaldehydes are measured in benzene solution. The angle which the dipole axis of the CHO group makes with the axis of rotation of the group is determined. The observed moments of the ortho-substituted benzaldehydes are compared with the moments calculated for free rotation as well as fors-transands-cisorientations of the -CHO group.o-Fluorobenzaldehyde exists mostly in thes-transconformation.o-Chloro-,o-bromo-ando-nitro-benzaldehydcs also exist in thes-transconformation; their observed dipole moments are even lower than the values calculated fors-transforms, indicating mutual induction of the ortho substituents. Though 2,5-dichlorobenzaldehyde is expected to have the same dipole moment as benzaldehyde, the observed moment is significantly lower due to mutual induction of the ortho substituents. 2,5-Dimethylbcnzaldehyde has, however, almost the same moment as benzaldehyde. The dipole moment ofo-methoxybcnzaldchyde is considerably higher than the values calculated for boths-cisands-transconformations. An explanation is given for this.o-Hydroxybenzaldehyde exists exclusively in thes-cisform due to internal H-bonding.