In a writing instrument including an ink reservoir and an air hole, the air hole is spanned by a barrier formed as a porous non-absorbent ink repellent membrane of substantially uniform pore size. The barrier is non-porously sealed, for example by direct heating or ultrasonic welding, along its entire periphery to the wall of the air hole, the barrier allowing air to pass freely therethrough in both directions, but preventing escape of ink therethrough and therearound. The barrier is preferably formed of polytetrafluoroethylene with a pore radius of not more than 10 .mu.m and a thickness of not more than 200 .mu.m. The ink for use in the writing instrument preferably has a surface tension of not less than 30 mN m.sup.-1, a viscosity of not more than 10 cp and a wetting angle with the barrier of not less than 100.degree..