Quaternary boron salts are used as initiators of polymerisation in adhesive compositions for bonding low surface energy substrates such as polyolefins. The quaternary boron salts are of the formula I ##STR00001## wherein R.sup.1 is C.sub.1-C.sub.10 alkyl, R.sup.2, R.sup.3 and R.sup.4, which may be the same or different, are, C.sub.1-C.sub.10 alkyl or C.sub.3-C.sub.10 cycloalkyl, phenyl, or phenyl-substituted C.sub.1-C.sub.10 alkyl or C.sub.3-C.sub.10 cycloalkyl, provided that any two of R.sup.1-R.sup.4 may optionally be part of a carbocyclic ring, and M.sup.+ is a metal ion or a quaternary ammonium ion. When M.sup.+ is a quaternary ammonium ion, the initiator may be used in a two-part composition having a (meth)acrylate monomer in both parts and the initiator compound in one part, the two parts being in a weight ratio of about 1:1.Particular examples of initiator compounds include sodium tetraethyl borate, lithium tetraethyl borate, lithium phenyl triethyl borate, tetramethylammonium phenyl triethyl borate, and tetra methyl ammonium phenyl tri-n-butyl borate, the last-mentioned of which is a novel compound.