Compounds of the type R2EH react readily with CF3C≍ CCF3 to give R2ECH (CF3)= C (CF3) H (E= N, P, As), and with CF3C≍ CH to give R2ECH= CHCF3 (E= N, As) and R2EC (CF3)= CH2 (E= As). The isomer distribution of the products is obtained from their nmr spectra and the mechanism of the addition reaction is discussed.