Prior period expenses is the expenses which is charged to the profit and loss account during the current year but the period of which pertains to earlier years. Normally it is disallowed but if it is crystalied in the current year then even though it belogs to earlier years , it is allowed fully in the current year. For example travelling expensss incurred by the employee of the company which pertains to previous years period but claimed in the current year. In such situation it will be allowed in the curent year because it is crystalised in the current year.. Similarly in case of salary revision with effet from earlier year , it will be allowed in the current year even though paid for the period pertaining to previous years.