The change from infinitives to clausal constructions is a character- istic of languages of the Balkan area - Greek, Bulgarian and Macedonian, Roumanian and Aroumanian, Albanian - which distinguishes them from their Indo-Europeancognates,andfrommostotherlanguagesoftheworld. It is therefore, interesting both as an unusual linguistic development and as a probable instance of syntactic interaction between geographically con- tiguous languages. The thesis is an attempt to examine, in as much detail as the scanty historical records permit, how this change took place in Bulgarian, and to consider in the light of their evidence the relative merits of the various explanations which have been offered for the change, whether as aBulgariandevelopmentorasaBalkanphenomenon. Thus,althoughatten- tion is given mainly to Bulgarian, it is hoped that the conclusions may beinsomepartacontributiontoBalkanstudies. Itisalsohopedthat the body of factual material presented, which is drawn from published sources either not previously studied from this point of view or not studied so fully, may be of use to other investigations of the history of Bulgarian. Thefindingsareheldtoaddfreshevidencetothoseinthe most detailed work on the subject, Mirchev's monograph of 1937, some of whose conclusions are disputed in the thesis. The object of research is defined as the Bulgarian language from the tenth to the seventeenth centuries, that is, from its first written re- cords to the date when its syntax had developed to approximately its modern state. For practical reasons Bulgarian and Macedonian are not dis- tinguished in the thesis, although their separate status as modern lan- guages is recognized. [Abstract continues in thesis]