AbstractSAXS measurements on dilute solutions of polyisobutene inn‐hexane (Mν= 25,600−3.55 × 106,c<53 g/liter) were carried out by combining conventional slit collimation with the new cone collimation technique. With a fractionated sample (Mν= 25,600;U<0.2) the radius of gyration (R= 48 Å), the cross‐sectional radius of gyration (Rq= 3.9 Å), the molecular weight per unit length (M/L= 22.8 Å−1), and the Porod persistence length (a*= 8.1 Å) are found. The persistence length appears to be dependent on the molecular weight. The partial specific volumeνof polyisobutene inn‐hexane also depends on the molecular weight according toν= 1.0