The invention relates to a phosphonium borate compound represented by Formula (I) (hereinafter, the compound (I)). The invention has objects of providing (A) a novel process whereby the compound is produced safely on an industrial scale, by simple reaction operations and in a high yield; (B) a novel compound that is easily handled; and (C) novel use as catalyst. Formula (I): (R.sup.1)(R.sup.2)(R.sup.3)PH.BAr.sub.4 (I) wherein R.sup.1, R.sup.2, R.sup.3 and Ar are as defined in the specification. The process (A) includes reacting a phosphine with a) HCl or b) H.sub.2SO.sub.4 to produce a) a hydrochloride or b) a sulfate; and reacting the salt with a tetraarylborate compound. The compound (B) has for example a secondary or tertiary alkyl group as R.sup.1 and is easily handled in air without special attention. The use (C) is characterized in that the compound (I) is used instead of an unstable phosphine compound of a transition metal complex catalyst for catalyzing C--C bond, C--N bond and C--O bond forming reactions and the compound produces an effect that is equal to that achieved by the transition metal complex catalyst.