A colourimetric vacuum air pressure indicator is described, based on the very low level of CO2 in air. The indicator uses the pH indicator dye, ortho-cresolphthalein, OCP, which is violet coloured in its deprotonated form and colourless when protonated. When the violet coloured OCP anion is ion-paired with the tetrabutylammonium cation, the product is readily dissolved in a non-aqueous solution containing the polymer ethyl cellulose to create an ink which, when cast and allowed to dry, responds to levels of CO2 well below that in air, i.e. << 0.041%; the indicator's halfway colour changing point is at 0.062 atm of air at 22 oC, which is interesting in that in food vacuum packaging the pressure in the pack is usually ca. 0.04 atm. The indicator can be used as a qualitative and quantitative indicator of vacuum air pressure. The latter requires the use of digital photography, coupled to RGB colour analysis, in the analysis of the indicator's colour. As with most CO2 indicators, the indicator's response is temperature sensitive, with H = 78 5 kJ mol-1. The indicators 90% response and recovery times to a cycle of vacuum and air were 16.2 and 2.7 min, respectively. The efficacy of the indicator as a vacuum-package integrity indicator for food packaging is illustrated and other potential applications are discussed briefly. This is the first reported example of an ink-based, inexpensive vacuum air pressure indicator.