Madrid, Barcelona, Athletic Bilbao and Osasuna are all being investigated for possible tax privileges, while Madrid are also the subject of another inquiry regarding the transfer of land with the City of Madrid.
The other three clubs involved, Valencia, Elche and Hercules, are being probed by the Commission over aid received from the state-owned Valencia Institute of Finance.
Here are the measures for which each club is being investigated, as announced in a statement by the European Commission:
Real Madrid, Barcelona, Athletic Bilbao and Osasuna
"Since 1990: Possible privileges regarding corporate taxation of Real Madrid CF, Barcelona CF, Athletic Club Bilbao, and Club Atletico Osasuna - These four clubs are exempted from the general obligation for professional football clubs to convert into sport limited companies. The effect of this exemption is that these clubs enjoy a preferential corporate tax rate of 25 per cent instead of 30 per cent applicable to sport limited companies."
"2011: Real Madrid appears to have benefited from a very advantageous real property swap with the City of Madrid. This swap was based on a re-evaluation of a plot of land at a value of 22.7million euros, instead of its earlier supposed value in 1998 of 595 000 euros."
"2009: State guarantee by the Valencia Institute of Finance for a bank loan of 75million euros from Bancaja (now Bankia) to Fundacion Valencia Club de Futbol, which was used to finance the acquisition of shares of Valencia CF by the Fundacion Valencia.
"2010 and 2013: The Valencia Institute of Finance increased its guarantee to Fundacion Valencia Club de Futbol by six million euros and five million euros, respectively, to cover overdue capital, interest and costs, stemming from defaulted payments of the guaranteed loan previously granted to CF Valencia."
"2010: State guarantee by the Valencia Institute of Finance for a bank loan of 18million euros from Caja de Ahorros del Mediterraneo to Fundacion Hercules de Alicante, which was used to finance the acquisition of shares of Hercules CF by Fundacion Hercules de Alicante."
"2013: State guarantee by the Valencia Institute of Finance for two bank loans of totally 14million euros, from CAM (nine million euros) and from Banco de Valencia (five million euros), to Fundacion Elche Club de Futbol, which was used to finance the acquisition of shares of Elche CF by Fundacion Elche Club de Futbol."