Addition of alcohols and phenols to O-allyl compounds (allyl ethers and acrolein acetals), catalyzed by [RuCl 2 (PPh 3) 3 ], was examined. Intramolecular addition of an OH group, leading to the formation of cyclic acetals and orthoesters, was also inves-tigated. As a result, a new, selective and convenient method for the synthesis of symmetrical and unsymmetrical (mixed) acetals and orthoesters was developed. Mixed (unsymmetrical) acetals are widely used in synthetic organic chemistry. 1–3 Also, in the fragrance 4,5 and pharmaceutical 6 industries, acetals are used both as intermediates and as end products. Acetals are recog-nized as good carbonyl protecting groups. 7,8 There are many methods for the synthesis of acetals 9–11 and ortho-esters 12 in the literature. However, selective syntheses of mixed acetals of type RCHOR 1 (OR 2) and orthoesters of type RC(OR 1) 2 (OR 2) is still difficult. Application of classical methods for the synthesis of these compounds is not satisfactory, as a concomitant transacetalization reaction leads to a mixture of symmetrical and unsym-metrical acetals and orthoesters, which can be difficult to separate. So far, only one report describing a selective method for the preparation of several mixed acetals 13 has been published. These compounds were obtained by reaction of dimethyl (or diethyl) acetals of type R 1 CH(OMe) 2 with TESOTf and 2,4,6-collidine and then by treating the salt obtained with R 2 OH. 13 A few attempts using transition metal complexes for the synthesis of mixed acetals have not been successful. Chang obtained various symmetrical and unsymmetrical acetals, in the reactions of R 1 OCH 2 CH@CHR 2 with methanol, catalyzed by cobalt complexes generated