Changes in sialylation of cell surface or plasma proteins are often associated with various cancers and other disease conditions. Provided are methods of detecting biomarkers of conditions associated with a change of sialylation status. Sialylated peptides are first isolated from biological and other samples by loading onto titanium dioxide (TiO2) or zirconium dioxide (ZrO2) stationary phase under acidic conditions in a solution comprising at least 20% organic phase and at least about 6.5 mM of substituted aromatic carboxylic acid, or, alternatively, at least about 11 mM short chain, non-aromatic, hydroxylated carboxylic acid. Sialic acid containing proteins can then be eluted from loaded stationary phase material by exposure to an alkaline solution having pH of 9.0 or greater, preferably at least 10.5. Sialylated peptides isolated by the methods provided can then be analysed by mass spectrometry to identify patterns of sialylation across a sialiome (the entire complement of sialic acid containing peptides in a biological sample) and/or to identify proteins in a sample that are sialylated or that show changes in sialylation status between two or more different samples. In preferred embodiments, sialylated peptides from control and patient samples can be differentially isotopically labelled and compared in a single mass sprectrometry experiment. Also provided are specific biomarkers of bladder cancer. The methods for isolating and analysing sialylated proteins can also by applied to phosphorylated proteins.