The crystal structures of trans-4-hydroxycrotonic acid (THCA), 4-o-chloro-phenyl-THCA (THCCA), and 4-p-nitrophenyl-THCA (THNCA) have been determined by single-crystal X-ray diffraction techniques, and refined by full-matrix least squares. THCA crystallizes in the monoclinic space group Cc with a=7.847(1), b=8.519(1), c=7.685(4) Å, β=109.66(2)° and Z=4; THCCA is triclinic, space group P-1, with a=7.878(2), b=8.621(1), c=7.653(1) Å, α=92.20(1)°, β=114.15(2)°, γ=94.40(2)°, and Z=2; THNCA is orthorhombic, space group Pna21, with a=7.488(1), b=19.666(2), c=7.143(3) Å, and Z=4. Final R-factors are 0.034, 0.045, and 0.031, respectively. The first two compounds present a semiextended conformation. The third structure is extended; however, for this last compound, empirical molecular mechanics calculations show that the semiextended conformation corresponds also to a low-energy state. © 1987 Plenum Publishing Corporation.