This paper describes a computer program which performs a particular type of grammatical/syntactic analysis—the assigning of structural boundaries between orthographic words in written English text. The Boundary Marker has been designed, in principle, as an analyser of unrestricted text and has been developed by using, as far as possible, authentic text as data for analysis. This paper first presents a brief overview of boundary marking as a method of syntactic analysis. It then describes how the program processes text and reports on the analysis of 10 000 words of text from the media. The paper concludes with a discussion of the advantages of a tightly focused analytic tool such as the Boundary Marker.