Three 1-phenylindolin-2-one derivatives, namely 6-chloro-1-phenylindolin-2-one (A), 4-chloro-1-phenylindolin-2-one (B) and 1-(3-chlorophenyl)indolin-2-one (C), all C14H10ClNO, have been synthesized and the structures of these compounds at room temperature (ART, BRT and CRT) and low temperature (ALT, BLT and CLT) have been determined. Crystal A at 295 K (ART) crystallized in the monoclinic space group P21/c and the phenyl ring of the unique molecule exhibits disorder over two positions. At low temperature, the disorder disappeared and crystal A at 90 K (ALT) crystallized in the triclinic space group P-1 with a doubled unit-cell volume and four molecules in the asymmetric unit. Density functional theory (DFT) calculations revealed a low oscillation barrier (0.24 kcal mol-1) of the phenyl ring of molecule A and the phase transition from the P21/c structure to the P-1 structure can be interpreted in terms of the freezing out of the two conformations at low temperature. Crystal B retains space group P21/c in the temperature range from 100 to 297 K. A kind of obtuse-cell to acute-cell change can be recognized; if the unit cell of BLT at 100 K is set to be a standard obtuse cell [β = 90.341 (2)°] and the cell is kept untransformed in the course of temperature changing, the cell of BRT at 297 K was found to be acute [β = 89.288 (2)°]. The molecules in structure C are packed in layers, with C-H...O hydrogen bonds between neighbouring layers.