A method for synthesizing oligonucleotides on a solid substrate. The method provides for the irradiation of a first predefined region of the substrate without irradiation of a first predefined region of the substrate. The irradiation of a second predefined region of the substrate. The irradiation step removes a protecting group therefrom. The substrate is contacted with a first nucleotide to couple the nucleotide to the substrate in the first predefined region. By repeating these steps, an array of diverse oligonucleotides is formed on the substrate.