The invention relates to a method of preparing a 5′ and 3′ modified library of template polynucleotides and also the use of the 5′ and 3′ modified library of templates in methods of solid-phase nucleic acid amplification. In particular, the invention relates to a method of preparing a 5′ and 3′ modified library of template polynucleotides which have common sequences at their 5′ ends and at their 3′ ends, wherein over-representation of “end” sequences of the primary polynucleotide molecules from when the 5′ and 3′ modified library is generated is greatly reduced or prevented.