We report on formation of a new surface covalent organic framework (SCOF) based on a sequential polymerization in two stages. The bifunctional precursor used, p-bromo-benzene boronic acid (BBBA), was vapor deposited in ultra-high-vacuum conditions on a Au(111) surface. In a first step the boronic acid moieties reacted to form trimers linked by covalent boroxine rings. In a second step, an Ullmann coupling reaction was thermally activated to form C− C bonds between the trimers, leading to the final polymeric network. By this procedure the growth mechanism was optimized and the polymerization reaction yield could approach 100%. A well-extended network was obtained, differing from the ideal honeycomb-like network by a substantial number of nonhexagonal pores, which are intrinsic defects to the polymer structure. A comparison of the two competing reactions provides new insights into the complex mechanisms governing growth of two-dimensional covalent networks.