Gas-phase electron diffraction (ED) data obtained at 25-26-degrees-C together with results from ab initio calculations have been used to determine the molecular structure and conformational composition of 2-methylpropanal, (CH3)2HC-C(H)=O, and 2-methylpropionyl chloride, (CH3)2HC-C(Cl)=O. For 2-methylpropanal earlier published values for rotational constants for several isotopes have also been included in the analysis. For both compounds the majority of the molecules have a gauche conformation where a CH3 group is nearly eclipsing C=O. Small amounts of a syn form where H is eclipsing C=O (C(s) symmetry) probably also is present in the gas phase. The results from this combined analysis for some important bond distances (r(g)) and valence angles (angle alpha) are as follows for 2-methylpropanal : r(C-H)methyl = 1.109(3) angstrom, r(C=O) = 1.226(3) angstrom, r(OC1-C2) = 1.513(2) angstrom, r(C2-C3) = 1.526(2) angstrom, r(C2-C4) = 1.537(2) angstrom, angle C2-C1=O = 123.8(9)degrees, angleC1-C2-C3 = 113.5(7)degrees, angle C1-C2-C4 = 109.2(9)degrees, angle C3-C2-C4 = 111.6(4)degrees, and alpha(g) (mole fraction of the low energy conformer) = 0.88(14). For 2-methylpropionyl chloride the results are as follows : r(C-H)methyl = 1.108(6) angstrom, r(C=O = 1.186(3) angstrom, r(OC1-C2) = 1.511(3) angstrom, r(C2-C3) = 1.534(3) angstrom, r(C2-C4) = 1.540(3) angstrom, r(C-Cl) = 1 804(4)) angstrom, angle C2-C1 = O = 127.3(7)degrees, angle C1-C2-C3 = 109.7(8)degrees, angle C1-C2-C4 = 109.9(8)degrees, angle C3-C2-C4 = 113.8(27)degrees, angle C2-C1-Cl = 113.6(5)degrees, and alpha(g) = 0.82(12). Parenthesized values are 2sigma. The results are compared with the ab initio values and with results for related molecules.