The invention provides novel labelled boronic acid conjugates of formula ##STR1## (wherein V is a reporter moiety; W.sup.2 is a bond or an organic linker moiety; W.sup.1 is a *SO.sub.2 NR.sup.2, *CONR.sup.2 or *CH.sub.2 N.sup..sym. R.sup.2.sub.2 group bound at the *-marked atom to the phenyl ring; R.sup.1 is hydrogen or an electron withdrawing substituent group; and each R.sup.2 independently is hydrogen or an optionally hydroxylated and optionally C.sub.1-6 -alkoxylated C.sub.1-6 -alkyl group) and salts thereof, e.g. for use in assays for cis-diols such as glycated blood proteins, having enhanced water-solubility and storage stability.