Three ¹⁵N-labelled trans-Pt(II) amine complexes with isopropylamine (¹⁵N-ipa), methylamine (¹⁵N-ma) and dimethylamine (¹⁵N-dma) have been prepared and characterized. 2D [¹H,¹⁵N] HSQC NMR spectroscopy was used to obtain the rate and equilibrium constants for the aquation of trans-[PtCl₂(¹⁵N-ipa)(¹⁵N-ma)] (¹⁵N-1), trans-[PtCl₂(¹⁵N-dma)(¹⁵N-ma)] (¹⁵N-2) and trans-[PtCl₂(¹⁵N-dma)(¹⁵N-ipa)] (¹⁵N-3) in 100 mM NaClO₄ solutions at 298 K. New ¹⁵N shift ranges for H₂N–Pt^II–N and HN–Pt^II–N groups are reported. Formation of the diaqua complex was not observed for 3 and accounted for <2% of the species at equilibrium for 1 and 2. The first aquation step is significantly faster for 2 (k₁ = 14 × 10⁻⁵ s⁻¹) than for the two complexes with the bulkier ipa ligand (k₁ = 5.5 × 10⁻⁵ s⁻¹ (1), 6.1 × 10⁻⁵ s⁻¹ (3)), but 2 is the least aquated of the three complexes at equilibrium. The pK_a values for the monoaqua adducts of 1–3 are similar (5.98, 5.85 and 5.91, respectively) and 0.4 pH units lower than the related cis complex cis-[PtCl₂(dma)₂], indicating a smaller proportion of more reactive aqua species will exist at physiological pH. The pK_a values for the diaqua adduct of 2 (4.59 and 7.98) are 0.3–0.6 pH units higher than those of 1 (4.31 and 7.30) and 3 (4.28 and 7.29), which have very similar values. The speciation profiles of 1–3, calculated on the basis of the calculated equilibrium and dissociation constants, indicate that <1% hydrolyzed species will exist under physiological conditions in cancer cells. The cytotoxicity of 1–3 (non-¹⁵N-labelled) was assessed in three cancer lines (SF268, MCF-7 and NCI-H460). The new trans-Pt(II) diamine complex 2 is more active than 1 and 3 in all cases and is more potent than cisplatin in the MCF-7 adenocarcinoma cell line.