A process for the preparation of halogenated squaraine dyes useful as sensitizers of the general formula 1 wherein X is a heavier halogen atom, or pharmaceutically acceptable derivatives.It comprises reacting bis (2, 4, 6, trihydroxyphenyl) squarine with a halogen solution or a halogen salt in an organic acid selected from glacial acetic acid under stirring at a temperature in the range of 50 - 80°C for a time period ranging from 1 to 5 hours, cooling the above reaction mixture, adding water to the reaction mixture, filtering and washing the resultant compound followed by recrystallisation to obtain halogenated squaraine dye.