AbstractMeasurements of the coefficient of thermal expansion on the spin‐liquid candidate κ‐(BEDT‐TTF)2Cu2(CN)3 have revealed distinct and strongly anisotropic lattice effects around 6 K – a possible spin liquid instability. In order to study the effects of a magnetic field on the low‐temperature spin‐liquid state, dilatometric measurements have been conducted both as a function of temperature at B = const. and as a function of field at T = const. While the 6 K anomaly is found to be insensitive to magnetic fields B ≤ 10 T, the maximum field applied, surprisingly strong B ‐induced effects are observed for magnetic fields applied along the in‐plane b‐axis. Above a threshold field of 0.5 T < Bc ≤ 1 T, a jump‐like anomaly is observed in the b‐axis lattice parameter. This anomaly, which is located at 8.7 K at B = 1 T, grows in size and shifts to lower temperatures with increasing the magnetic field. Although the anomaly bears resemblance to a first‐order phase transition, the lack of hysteresis suggests otherwise. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)