Recent advancements in wearable inertial sensor technology have enabled extensive research into activity classification. However, the distinction between the sitting and standing activities using trunk-worn wearable monitoring systems remains challenging, calling for additional sources of information such as altitude change. The aim of this study is to demonstrate the suitability of barometric pressure, an absolute estimate of sensor altitude, for distinguishing between sitting and standing (STS), as part of a wearable system. This barometric pressure sensor, the MS5611-BA01 from Measurement Specialties (CH), was integrated in a comfortable, miniaturized wearable inertial sensor device suitable for improved long-term activity recognition. The time evolution of the different modalities is displayed in Figure 1. A pilot study was conducted with 7 healthy volunteers (6 Males and 1 Female / Age: 27.8±2.1 years / BMI: 23.9±4.5 / Height: 1.80±0.075 m). Subjects were recorded performing a set of activities of daily-life including static postures (e.g., sitting or standing), postural transitions (e.g sit-to-stand, stand-to-sit) and dynamic activities (walking, climbing up/down stairs) in both indoor and outdoor real-world conditions. It enabled discriminating actual STS transitions from stationary postures with accuracies of 99.5% and 98% using averaging windows of Δt_average =4.16s and Δt_average =0.32s respectively. It is also capable of completely discriminating between Sit-to-Stand and Stand-to-Sit transitions.