From the Wikipedia page 
Pontoise (French pronunciation: [pɔ̃twɑz]) is a commune in the northwestern suburbs of Paris, France. It is located 28.4 km (17.6 mi) from the centre of Paris, in the "new town" of Cergy-Pontoise.
Pontoise is the official préfecture (capital) of the Val-d'Oise département, although in reality the préfecture building and administration, as well as the department council (conseil général), are located in the neighboring commune of Cergy, which is regarded as the de facto capital of Val-d'Oise.
Pontoise is also the seat of the Arrondissement of Pontoise. The sous-préfecture building and administration, unlike the préfecture, are located inside the commune of Pontoise.