Seventy contract cleaners at one of Wales' biggest steelworks are striking in a row over pay.
The Community union, which will picket at Tata's Port Talbot plant during the two-day stoppage, want an improved offer from contractor OCS.
The union said most if its workers at the site earned the minimum wage and it had been trying to secure a pay rise since last June.
OCS said it regretted the strike and it was offering an above inflation rise.
The unon said that OCS currently pay their workers significantly less than any other contractor on the Tata Steel site, but this has been denied by OCS.
Susan Lewis of Community Union said: "It is unfortunate that we have got to this point - our members do not take industrial action lightly, but OCS workers do some of the toughest and dirtiest jobs on the steelworks with little reward - it is just not fair."
But OCS said: "During our ongoing negotiations we have put forward a pay offer greater than the rate of inflation, yet unfortunately the union has chosen to withdraw from this process."
A Tata Steel spokesman said: "This matter is between OCS workers and the OCS management but we are hoping they can resolve it".