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Gas-phase electron diffraction (ED), together with ab initio molecular orbital calculations, have been used to determine the structure and conformational composition of 1-chlorobutane, 1-bromobutane, and 1-iodobutane. These molecules may in principle exist as mixtures of five different conformers, but only three or four of these were observed in gas phase at temperatures of the ED experiments, 18~ 18~ and 23~ respectively. The observed conformational compositions (1-chlorobutane, 1-bromobutane, and l-iodobutane) were AA (13 + 12%, 21 + 14%, 19 + 17%), GA (60 d-13%, 33 + 32%, 17 + 31%), AG (12 + 16%, 8 + 12%, < 1%), and GG (12 + 16%, 38 5: 34%, 64 5: 31%). A and G denotes anti and gauche positions for the X--CI--C2--C3 (X = C1, Br, I), and the C~--C2--C3--C 4 torsion angles. The results for the most important distances (rs) and angles ( / ~) from the combined ED/ab initio study for the GA conformer of 1-chlorobutane, with estimated 2a uncertainties, are r(C~--C2) = 1.519(3) .~,, r(C2--C3) = 1.530(3) tk, r(C3--C4) = 1.543(3) /~., r(Ci--Cl) -1.800(4) .A.