Very few papers deal with comprehension of mechanisms leading to the formation of layered double hydroxide (LDH) compounds which are usually obtained by coprecipitation at constant pH value. In this paper, we study the influence of pH values of coprecipitation on the formation of LDH phases, their crystallinity and chemical composition, and then discuss about LDH formation mechanisms, using a synthesis route with varying pH. We clearly show that LDH phases can be obtained by such a method, at pH values from 10 to 13.2. A rich aluminium specie is firstly obtained which tends to continuously incorporate Mg until LDH species with the targeted Mg/Al ratio are obtained.