Addressing a hearing in Parliament on Tuesday, the Czech official told MEPs that "efficient" enforcement of rules was "as important" as the legislation itself.
Jourová, who is responsible for justice, consumers and gender equality, was speaking at the latest meeting of a special inquiry set up by Parliament in the wake of the Panama Papers scandal.
She outlined details of various initiatives taken by the Commission, including the fourth anti-money laundering directive which, she said, is due to take effect in June next year.
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The official told the meeting that the Commission would "keep a close eye" on those member states which fail to implement the directive once it comes into force.
She said, "It is vitally important that such rules are applied and enforced correctly by member states."
The Commission had infringement action in the past and she cited as an example the fact that it had referred six member states to the ECJ for failing to implement similar EU-wide anti-money laundering measures in 2007.
"The court found that five of these had failed to transpose the directive," she pointed out.
Recent cases, such as the Panama Papers furore, had highlighted the need for such measures, she said, adding that tax evasion was a "very serious phenomenon" which "needs to be eradicated."
She said, "Tax avoidance and evasion deprives national budgets of millions of euros in revenue and we shall continue to take action against such activity."
The Panama Papers scandal only came to light as a result of whistleblowing. The Commissioner also backed efforts to provide more legal protection to whistleblowers.
She said, "The need for whistleblowers has been clearly demonstrated by recent events and we aim to strengthen protection for such people in the fourth anti-money laundering directive."
The EPP group in the European Parliament has previously criticised the "foot-dragging" of many countries when implementing existing international standards against money-laundering.
"Many countries have failed to effectively implement existing anti-money laundering rules. Before we call for new measures, the focus must be first on the effective implementation of the existing standards", said Dariusz Rosati, EPP group spokesperson on the committee.
The committee investigating the Panama Papers scandal has held several meetings but Tuesday's was the first where a Commissioner has appeared.
The committee is mandated to establish why banks, business lawyers, audit firms and other finance companies managed to build up a global network of letterbox companies without being punished.
It is expected to complete its work next year.