Q: Do Barcelona Taxis generally accept common credit cards (Master Card)?
A: Some taxis do have a credit card processing terminal in their taxi however the great majority of taxis do not have credit card processing facilities. It is therefore recommended that you always ensure to take enough cash in euros to be able to pay for the taxi fare.
If the taxi does accept credit cards then ensure that you have a visa card as all credit card terminals will process visa (and usually MasterCard) AMEX is not generally accepted as this requires a special contract with the credit card supplier.
If you really want to pay your taxi fare with a credit card ask the driver before you accept the ride and show him the credit card you wish to use to pay the taxi fare with. He will be able to tell you.