Separation and determination of lanthanides, Th and U is of great relevance in different fields of science and technology. Reversed-phase high-performance liquid chromatography (RP-HPLC) using alpha-hydroxy isobutyric acid (alpha-HIBA) as an eluent on reversed-phase column modified to cation exchanger has been reported to achieve the separation. However, under those conditions, Th and U are eluted amongst lanthanides, making their quantification difficult due to overlapping with some of the lanthanides peaks. In this work, different chromatographic parameters (concentrations of eluent and ion interaction reagent, pH, etc.) were studied systematically to arrive at optimum chromatographic conditions. Using the dual (concentration and pH) gradient conditions, lanthanides, Th and U could be separated in 11 min by RP-HPLC with sequential elution of Th and U after the elution of all the lanthanides. The separation methodology was tested using SY-3 rock sample for the separation and determination of lanthanides, Th and U. The method allows an accurate determination of these elements in a single run using a single column. Also, the method is fast and cost-effective compared to the reported methods.