Letras are financial assets issued at a discount (with an implicit yield). The difference between the amount earned on the Letra's sale or redemption and the amount paid in its purchase is treated as capital income and accordingly subject to personal income tax.
Any yield generated between the acquisition and the sales price or the amortisation of Treasury Bills, whatever their term may be, shall be taxed at 19% for taxable bases up to 6,000 €. Taxable bases between 6,000 € and 50,000 € shall be taxed at 21%, and taxable bases exceeding 50,000 € shall be taxed at 23%.
No withholding tax is applied to such income, whoever the recipient.
Letras del Tesoro