The reaction of GaBu(3)(t) with phenylphosphonic acid results in the formation of the unusual phosphonate bridged dimer, [Bu(2)(t)Ga{mu-O2P(Ph)OGaBu(2)(t)}](2), whose molecular structure consists of two four- and two three-coordinate gallium centres; while [Bu(2)(t)Ga{mu-O2P(Ph)OGaBu(2)(t)}](2) exists as the trans isomer in the solid state, it undergoes a trans to cis isomerization in solution, Delta H = 7 +/- 1 kJ mol(-1) and Delta S = 25 +/- 4 J mol(-1) K-1.