The Second Pension Income (€16.50c per month) is most likely administered by one of the major insurance companies, i.e.Irish Life,Zurich etc. As your Standard rate cut off point (S.R.C.O.P) & Tax credits are allocated to your Main pension income, there are none allocated to the 2nd one. In that case the administrator is obliged to deduct, tax & usc on an "emergency basis". This is why the Revenue would like to allocate a portion of your SRCOP/Tax credits to your second pension. When the administrator receives this Certificate,then the emergency tax usc are removed and said tax & usc is deducted accordingly. Try this,assume the 2nd pension has a portion of all the credits/SRCOP allocated, now check this pay slip to see what tax/usc was deducted,write it down.Similarly do the same with the main pension income,write down deductions,now add Both tax and both usc's deductions,write them down. This part should confirm(Monthly based calculation) if the deductions are being made correctly. Add all of the incomes,check and ensure they are below SRCOP of €2,942.00c.(Per Month)If it is calculate tax @ 20% of the income, Now check both tax credits total them,and subtract from the tax calculated.(20% of all incomes )This calculation should equate with the tax deducted from BOTH payslips. If this is not the case, re-check all of your SRCOPS/Tax Credits. This calculation assumes it will be for a single taxpayer, and you receive your income on a monthly basis. The observation that tax increases with the Main pension and reduces with the second one,means that the allocation of SRCOP/Tax credits is most likely working correctly. I hope this is of assistance.