Single crystals of KTiOPO4:Ln ([n Yb, Nd, Ho, Er and La) were successfully grown by the top seeded solution growth (TSSG) method using potassium polyphosphate as a flux. A series of rare earth metal (Yb, Nd, Ho, Er and La) doped KTP crystals was subjected to high electric field and hydrogen annealing treatment at 973 K temperature. The vital role of alkali ionic conduction and Ti3+ in colouration of KTP has been studied by permittivity and ac conductivity studies. The hydrogen anneal- and UV bleach-induced grey colouration was studied by absorption and transmittance characteristics. The structural changes induced by grey tracking on KTP:Ln were studied by Raman shift analysis. The oxygen vacancies or Ti3+ centres were created by various treatments and the degree of colouration in the crystal was evaluated. The mechanisms of colouration in the crystal and resistance behaviour of grey colouration by lanthanide doping were studied. (C) 2015 Elsevier B.V. All rights reserved.