A process for the preparation of dry thermally stable chlorinated polyvinyl chloride, said process comprising: i. introducing polyvinyl chloride and water in an air tight vessel to form a slurry; ii. introducing chlorine gas into said air tight vessel till a chlorine pressure in the range of 1 kg/cm2 to 5 kg/cm2 is attained, and stirring the slurry and chlorine gas to obtain a reaction mixture; iii. irradiating said reaction mixture using radiation having a wavelength in the range of 390 nm to 730 nm, while heating at a temperature in the range of 50 °C to 90 °C to obtain a product mixture comprising chlorinated polyvinyl chloride and water solution including dissolved unreacted chlorine gas and hydrogen chloride; iv. filtering said product mixture to obtain a first mass comprising chlorinated polyvinyl chloride, and a first residuary water solution; v. adding water to said first mass under stirring for extracting unreacted chlorine gas and hydrogen chloride to obtain an aqueous suspension, followed by pressure filtering said aqueous suspension to obtain a second mass comprising chlorinated polyvinyl chloride, and a second residuary water solution; vi. passing nitrogen through said second mass at a temperature in the range of 50 °C to 85 °C to extract away said second residual water solution to leave behind a third mass having water solution content less than 0.5 wt; and vii. agitating said third mass in a high-speed mixer together with a neutralizing agent, and with a stabilizer and a processing additive to obtain dry thermally stable chlorinated polyvinyl chloride having thermal stability in the range of 1000 seconds to 1500 seconds; wherein: • said neutralizing agent is calcium hydroxide, • said stabilizer is synthetic hydrotalcite; and • said processing additive is a copolymer with acidic groups. i. introducing polyvinyl chloride and water in an air tight vessel to form a slurry; ii. introducing chlorine gas into said air tight vessel till a chlorine pressure in the range of 1 kg/cm2 to 5 kg/cm2 is attained, and stirring the slurry and chlorine gas to obtain a reaction mixture; iii. irradiating said reaction mixture using radiation having a wavelength in the range of 390 nm to 730 nm, while heating at a temperature in the range of 50 °C to 90 °C to obtain a product mixture comprising chlorinated polyvinyl chloride and water solution including dissolved unreacted chlorine gas and hydrogen chloride; iv. filtering said product mixture to obtain a first mass comprising chlorinated polyvinyl chloride, and a first residuary water solution; v. adding water to said first mass under stirring for extracting unreacted chlorine gas and hydrogen chloride to obtain an aqueous suspension, followed by pressure filtering said aqueous suspension to obtain a second mass comprising chlorinated polyvinyl chloride, and a second residuary water solution; vi. passing nitrogen through said second mass at a temperature in the range of 50 °C to 85 °C to extract away said second residual water solution to leave behind a third mass having water solution content less than 0.5 wt; and vii. agitating said third mass in a high-speed mixer together with a neutralizing agent, and with a stabilizer and a processing additive to obtain dry thermally stable chlorinated polyvinyl chloride having thermal stability in the range of 1000 seconds to 1500 seconds; wherein: • said neutralizing agent is calcium hydroxide, • said stabilizer is synthetic hydrotalcite; and • said processing additive is a copolymer with acidic groups. • said neutralizing agent is calcium hydroxide, • said stabilizer is synthetic hydrotalcite; and • said processing additive is a copolymer with acidic groups.