A manual in situ crystallization technique is described, for application on a κ-geometry area-detector instrument. The technique has been applied to grow crystals of some linear alkynes: 1-heptyne, 1-octyne, 1-nonyne and 1-decyne, Cn H2n–2 (n=7, 8, 9, 10). The structures with odd n (1-heptyne and 1-nonyne) crystallize in space group Pca21. The structures with even n (1-octyne and 1-decyne) crystallize in space group P21/c, with two molecules in the asymmetric unit (Z′=2) and an approximately orthorhombic metric. All of the structures contain layers of molecules with a square unit mesh of dimensions 5.7×7.2 Å, identical to the layers present in the previously published structures of 1,7-octadiyne and 1,9-decadiyne. The structures differ where the methyl groups meet, giving systematically greater packing efficiency for 1-heptyne and 1-nonyne, compared to 1-octyne and 1-decyne. This systematic alternation in crystal density is correlated with an alternation in melting points.