13th month and bonuses: how they are taxed
December payslips disappoint because bonuses are taxed at your marginal rate, not your average rate. That is normal. What is not normal is a bonus missing, pro-rated wrongly, or taxed twice.
Is a 13th month compulsory?
Not by law in Luxembourg. It comes from your contract, a collective agreement or company practice, and it is usually pro-rated for the months worked in the year. Check the pro-rating against your start date.
How it is taxed
Bonuses are 'rémunérations non périodiques': social contributions apply at the normal rates, and income tax is computed on the bonus as extra income for the year, which lands at your marginal rate (often 30% to 42% plus the 7% surcharge). The net you keep from a bonus is therefore typically 55% to 65%.
Common errors
Bonus paid at the wrong pro-rata; bonus included in the pension base twice; contribution ceiling not applied for high earners in the bonus month; the bonus pushed the CIS below what the card allows; a 'prime' line with no description at all.
Cross-border note
French, Belgian and German residents pay Luxembourg tax on the bonus like on the salary. In France, the full Luxembourg income is declared and the rate effect is handled through the tax credit method.
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Questions people ask
My bonus was taxed at 40%, is that a mistake?
Probably not: the slip taxes extraordinary income at your marginal rate. The annual return settles the final amount.
Related guides
Indicative rules as of October 2026. Not tax or legal advice.