The subtle experimental energy difference between isomers of 1,2-dibromoethenes (0.090 +/- 0.240kcal mol(-1)) has been confirmed by high level MO calculations: the cis isomer is more stable by 0.04 kcal mol(-1) than the trans isomer at the MP4(STDQ)/6-311++G(3df,3dp)//MP2/6-311++G(3df,3dp) level. It was strongly suggested that the most important origin of the cis-effect is the stabilizing hyperconjugative (charge-transfer) interaction of Br lone pairs with the pi(C=C)* antibonding orbital.