Urey‐Bradley force constants (UBFC's), calculated by a least‐squares method, were obtained for ethylene, vinyl bromide, vinylidene bromide, trans‐dibromoethylene, cis‐dibromoethylene, tribromoethylene, and tetra bromoethylene. Even with complete isotopic data it was found impossible to determine all the force constants for an individual bromoethylene; moreover, from the viewpoint of giving stability to the calculation, values have been supplied by transfer from other molecules within this series and these force constants have been constrained in the least‐squares refinement. By inspection of the correlation matrix, the uncertainties in the various force constants were, in many cases, found to be intimately related, and, it was usually found unnecessary to constrain both force constants of an indeterminate pair; in this manner we have kept the number of transferred force constants to a minimum. A comparison of the remaining UBFC's shows that they are moderately transferable. Although the flexible bond parameter SHH was found to be significant (but not transferable) in those molecules containing hydrogen atoms trans to one another, i.e., ethylene, vinyl bromide, and trans‐dibromoethylene, its value in cis‐dibromoethylene was small and made no significant contribution in explaining the observed frequencies. A potential function which includes a quadratic cross term between trans‐hydrogen bending coordinates was found to be adequate throughout the series and moderately transferable.