Introduction: Gafchromic EBT2 is being used for radiation dosimetry including photon, electrons and recently in proton beams, based on its color changes from transparent to blue due to polymerization. It is critical to understand the temporal response of the polymerization after irradiation to proton beam. Materials and Methods: Five small pieces cut from one sheet (lot no. A08060905A) were irradiated with different doses (2 - 10 Gy) using a clinical proton beam in reference condition (range 16 cm, spread-out-bragg-peak 10 cm). An EPSON flatbed 10000XL was used for digitizing 48 bit color RGB images. The films were kept in the same position on the scanner for each scan. Within a 6-day period, measurements were taken every 3 minutes for the first hour, every 5 to 30 minutes for the first 1-12 hours and then randomly after 12 hours. Red, green and blue frequency read out of the scanner for each film was analyzed in ImageJ software. Results: A mathematical function (netOD =α + β (1-exp (- t/Ta)) + γ (1-exp (-t/Tb)) can be used to describe the temporal response. The polymerization is time dependent with two time constants, Ta (25 minutes) and Tb (1000 minutes). α, β, and γ are dose dependent parameters. The percentage of OD change per 10 minutes remains less than 1% after the first hour. A 7%-11% increase in netOD was observed within the first 12 h when irradiated within 10 Gy, comparable to 9% reported for EBT films. Conclusions and Discussion: The post-irradiation rapid polymerization effect is observed on EBT2 film with the growth of OD. This characteristic can be described by a double exponential function. It is recommended to apply this function for temporal coloration effect to improve the accuracy for radiation dosimetry using EBT2 film in proton beam.