The paper discusses the syntax of clustering clitics in the history of Russian and Bulgarian. Modern Russian no longer has clusters, while Modern Bulgarian and Old Russian differ regarding the placement of clustering present BE-auxiliaries: Old Russian invariably puts 1-2p. present forms of the BE-auxiliary after pronominal clitics (PRON―AUX2), while Modern Bulgarian puts all present forms of the BE-auxiliary except for 3Sg. before them (AUX1―PRON―AUX2). Bulgarian-Valachian diplomas (XVth century) mostly pattern with Old Russian and place BE-clitics in AUX2, cf. понеже им се сꙋ ꙋзели, нꙋ им ест ꙋчинил напаст, while sporadic examples with BE-clitics in AUX1, cf. како сꙋ се и ѡт прежде хранили can be explained as a transition to the Modern Bulgarian/Balcanic Slavic type. The Old Russian system cannot be directly identified with the initial Old Church Slavonic/Old Bulgarian system, since Old Russian lost non-zero 3p. forms of the BE-auxiliary in the pre-written period.