The woman, who was walking on Berrisford Road in Market Drayton when she was attacked, was taken to hospital for treatment on injuries to her hands.
Police said two men rode past her on bikes and threw a chemical substance at her.
The attack took place at around 10.45am on Thursday.
Chief inspector Mark Reilly said: "This was undoubtedly a traumatic experience for the victim and we are keen to speak to anyone who may know these men or have seen the incident.
"We understand this is concerning for the community and I would like to reassure residents that we are taking this incident very seriously, We believe at this time that the substance was a chlorine mixture and poses no further threat to the public."
Anyone with information is urged to contact PC Evans on 01630 657 155 or call Crimestoppers on 0800 555111.